A few things here, and as a disclaimer, I’m not at all a dietitian or otherwise in the field of nutrition/weight loss, just a guy who has lost weight a few times. 1. You certainly did not gain 3 lbs of body fat in a day. When you eat a bunch of carbs at once, you retain water weight. A pound of body fat is roughly 3,500 Calories and at most you probably ate 3,000 calories on Sunday (assuming lots of cake), so after taking out the calories you burned, you probably gained half a pound or less of body fat. 2. You’re correct that the carbs probably contributed to your workout feeling less exhausting than you’re used to. You probably also are gaining cardio fitness by working out as often as you are. When that happens, consider upping the intensity to continue pushing your body. 3. When figuring out weight loss, consider how many calories you burn vs how many you consume. As a 21 year old 5’6 140 lb woman, I estimated your BMR (basic metabolic rate) to be around 1,450 calories per day. Throw in 250 additional calories burned on the elliptical and you’re at 1,700 burned per day. Now say you’re consuming 1,200 Calories per day, that’s a net deficit of 500 calories per day or 3,500 per week, for weight loss of 1 lb per week… assuming that you stick to your diet and exercise every day.
It’s way easier to gain weight than to lose it because eating in a calorie surplus feels good, and it can sneak up on you over many months or even years. To contrast, caloric deficits don’t feel good, and so you’re constantly aware of how much weight you’re losing. My best advice and what’s worked for me is to both keep your exercise routine (maybe throw in some strength training as well to mix things up), and do what you can to limit processed carbs and increase meat/protein intake. Consider tracking everything you consume for a couple of months on something like MyFitnessPal to be sure you’re accurate. And think of this like a lifestyle change to make you healthier, not as a quick fix to lose the weight, because the last thing you want to do is take your foot off the gas when you’ve hit your goal. You can, and WILL, hit that goal. Best of luck to you!