I've had a similar issue myself this week. Despite being on 1000 (net) calories, and exercising regularly, I haven't lost any weight. I then worked out that I actually wasn't eating enough.
Is Exante a low calorie diet / meal replacement diet? I'm not familiar with it. If so, then your body is probably reacting to the change of actual food.
You do seem to be on a very low amount of calories for your activity level (unless you're talking about being on 1300 net calories - i.e. you'd burn 500 swimming and eat 1800 then 1800-500=1300) and this can also stall weight loss. If you're eating 1300 calories and then burning 500, then you're only consuming a net of 800 calories which is very low and whilst you'll lose weight, you'll also lose lean muscle mass as well as fat which will slow your weight loss long-term. Muscle mass = higher metabolism = faster loss for what you do eat
Basically, you can work out how many calories you should be on (as a *typical* person - I'm not accounting for people who are severely disabled or have issues with metabolism) by calculating your BMR (there's a calculator online somewhere, have a look through google) - your basic metabolic rate. This is the number of calories you'd need to eat if you basically stayed in bed all day, to maintain your weight. Then you multiply your BMR by a number (given to you by the Harris Benedict equation - as you're exercising 5 days a week it would probably be about 1.55 for you) - this would be the number of calories you need to consume in a day to stay the same weight as you are now, at the same amount of activity. Reduce this by about 500-700 to work out how much you should be eating daily in order to lose weight (between 1 and 1.5lbs a week), but try not to reduce by more than 700, or go below 1200 net calories (calories AFTER exercise is taken away).
I'm not a qualified nutritionist, I hope to study towards it (or personal training) next year, so this information is very general and doesn't account for any medical *quirks* you may have - in which case you may want to talk to a GP.
Hope this helps! Best of luck! If you need any help with calculations, just PM me.
