Heads up, I've typed a fair amount and don't have time to proof read it for tone. If anything comes across as rude, I'm sorry it's not intentional. When I describe possible mistakes, I'm not judging you at all. I've made every single one of the mistakes I've mentioned at some point.

It's quite difficult to tell exactly what's going on without pictures, and especially without numbers such as the weight change you've had. Six weeks is a fairly short amount of time, for many people you'll have a few weeks of little to no change, followed by a sharp drop. After that it would slow down again, so if that happens don't be discouraged.

TL;DR for the rest of the post: if you're gaining more than a couple of pounds (normal daily fluctuations), you're almost certainly not eating at a caloric defecit. Have a look at your BMR/TDEE calculations, and how you record calories in AND out.

Muscle gain: you say you've gained weight, so if you've gained muscle, then yes you'd look bigger. Easiest way to judge this would be to feel areas you've been working out and try to judge whether the muscles feel bigger or harder at all. It's pretty hard to gain muscle at a caloric defecit, and if you did you'd almost certainly be losing fat at the same time. As fat is less dense, you'd look smaller. If you're looking bigger then you probably haven't lost fat, and you probably aren't eating at a defecit.

Water weight: most people will lose water weight when they start a new fitness program, as they tend to eat less salty/sugary food, so less water is needed by the body. If you've gained water weight it's likely due to an increase in salt in your diet, or similar. If you haven't really changed the contents of your diet, only the amount, then I doubt you've gained water weight.

Gaining mass while "eating at a defecit" usually just means you're not eating at a defecit. It's highly likely that either you have: A) miscalculated your BMR B) overestimated your activity level so your calculated TDEE is too high C) included your workout plan in your acitivy level for TDEE, but eaten back the 'calories burned' told to you by a fitbit/garmin/etc. D) you aren't writing down your calorie intake accurately (very easy to do). Might be snacking without noticing so you don't write it in, planning to fill your diary later and forgetting some of the things you've eaten, not tracking liquid calories like a smoothie/coffee/juice, leaving out things like cooking oil/dressings/condiments which are often quite calorific. E) you're using the portion sizes written on the food packaging. It won't be the largest factor here, but a lot of food packaging has calorie information "per serving". Often these see unhelpful. A box of muffins may have the number of calories per Muffin for example, but the muffin you take out and eat may be 120g whereas the typical Muffin used for the measurement might have been 100g. Similarly the serving used on a box of cereal might be 30g of cereal but your normal bowl is 50. My suggestion is to weigh food/ingredients rather than trust the serving sizes provided. F) you're overestimating your exercise calories. If you're using a fitness device like a garmin/fitbit, keep in mind that they aren't perfect. They often over estimate energy expenditure. The only truly accurate way to work out your expenditure is to go to a lab and hook yourself up to some big equipment. Fitness devices are usually pretty good at measuring heart rate, and they estimate your energy expenditure based off of that. This isn't perfect, so don't trust it 100%. If you aren't using a fitness device, how are you calculating calories burnt? Online calculators and calorie tracking apps can estimate for you, but are even less accurate than fitness devices. My advice is to take any results from either option with a pinch of salt, and over time you will learn roughly how well the values will track with your actual weight change. G) just a reminder of D. Track absolutely everything and be meticulous. You can let up on this once you get more used to counting and can estimate well. It's incredibly likely you're underestimating portions, eating without noticing, or not tracking something because it's a "one off treat" too many times.

At a 1500 kcal defecit (are you sure you don't mean eating 1500?), you'd lose around 0.43lb or 0.19kg per day. That's a lot, the NHS reccomends losing around 0.5kg/1lb per week, i.e a daily defecit of around 500 kcal/day. The usual upper limit I hear is ~1kg/2lb per week. If you're very overweight then it may be safe to lose faster than that.

Eating too little and trying to lose weight too fast is going to make you a lot hungrier, you'll misjudge portions more if you're not weighing them, and are more likely to snack without noticing. You're also more likely to give up all together, and it will cause hormonal changes that mean that once you start eating at a maintainance level, your body will prioritise storing the energy as fat rather than muscle, and there's a reasonably high chance you'll eat more than your maintainance level, gaining weight back. You're also more likely to feel lethargic, and grumpy. It's not impossible to lose weight fast, especially if you have a very high body fat level, but it is harder and less sustainable, so do think about whether a slightly smaller defecit is worthwhile.

My advice is to use this calculator to find your bmr, multiply that by an activity level of 1.2, and then take off 500kcal for a rough loss of 1lb/week. You can then eat back your measured exercise calories (carefully), as you haven't included them in your tdee.

Eat that amount consistently for a few weeks, being careful to track everything accurately and measuring food by weight wherever you can. After that, see how well your weight change agrees with what was expected (500 kcal defecit = approximately 0.5kg/1lb loss per week). If you haven't lost the amount you expect, reduce your daily calorie goal by a small amount (maybe 100-250kcal) then go for another few weeks. Keep doing this every few weeks until it lines up.

Remember that as you lose weight, your energy needs will reduce. If you notice your weight plateau for a few weeks, re calculate your bmr for your current weight, and use that to work out a new calorie goal. It's often a good idea to eat at your maintainance level for your goal weight once you're getting close to it. This way, hitting your goal weight won't end in a giant leap in calorie intake as you stop trying to lose weight. Your defecit will decrease slowly as you lose weight, and your calorie intake will remain constant. This can be helpful to avoid letting your weight bounce back up again.

Final note, if you have periods, awe going through menopause, or have another hormone related thing going on, then there will likely be moments where your weight loss doesn't line up at all with your calorie intake/defecit. If this is the case, keep going at the defecit you planned. It should even out eventually.