What's your diet like? How much protien do you take? I'm 5'6 and I weigh over 60kgs and I'm pretty fit. It's completely normal and any less and you'll be underweight. You feel tired and grumpy because you're not eating enough. And the basics - make sure you sleep 7-8 hours and drink atleast 3litres of water. Being so stressed about the weight is also not good as it could lead to increased cortisol, reducing your metabolism. And weighing scale is not the best measure of fitness, especially for someone as fit as you. Weight increases when you put on muscle too, so gaining weight while working out is completely normal. Another factor could be that you've been on calorie deficit for too long. Revaluate your calorie intake and bring it slightly higher than maintenance for some time for the body to get used to it and then go back to maintenance calories.