I started swimming just over four weeks ago. For the first week I swam 1,250 meters a day for 6 days. I injured my back and couldn't swim for a week and a half. In that time my weight dropped from 11st 10lbs to 11st 6lbs. I have now been swimming again for a week and a half (doing the same amount) and my weight has rocketed up to 11st 12lbs. I know you need to allow for muscle development but surely not so much so quickly. I feel fitter and I am still a smaller size but my BMI is just over a normal weight figure. Can anyone advise me?

You can't be doing anything wrong especially if you're a smaller size- prob just muscle. BMI is baloney anyway - it tells me I'm underweight and there's no way in a million years...!!
Keep swimming!
Jess xx