You might be under eating. Low calories to that extent means not enough protein and carbs to use during exercise. Have you talked to a dietitian or personal trainer? Get an eating plan with complimentary exercises put together.
Swimming is really good, but specifically doing fast slow. So I do ten fast strokes and then five normal carry that for as long as you can. Rest in between laps when u need to.this is if you like swimming tho. :)
Start your day with a cold glass of water excersise at least an hour i like to go jogging you can split it up half an hour in the morning and half an hour in the evening some walking in the midfle of it would do great too eat slower and get smaller plates it tricks your brain.Hope I helped! :)