I felt the same... I felt heavy trying to swim with my belly im 36+5
I know I went a month ago and I was 7 months I felt super heavy when I was getting out and it felt like allot of pressure
I don't really know, swimming felt amazing when I was early along! Especially for back pain! I am 33 weeks now and I would love to go swimming to ease some pain! But maybe the extra weight is what is bothering you.
Probably because you've gotten used to carry in extra weight but in water your light and no weight..? Just a guess xx