Don't you feel like you're losing muscle when you swim?

I've been taking swimming correction classes for two months now.
It's so much fun that I feel like going every single day.
Since I'm not eating much after swimming, I definitely feel like my belly fat and extra flab are disappearing.
But the problem is, one day I looked in the mirror and felt like my thighs had become very thin.
Is it common to lose lower body muscle when swimming?
It also seems like people who swim consistently generally have thin legs.