My mum said something similar when we were looking at a magazine article once about swimsuits for every body.. I just pointed out to her that we have moved on since she was pregnant and lots of women show their bellies while at the pool/ beach. I don't have it exposed all the time. I also pointed out how expensive it is to buy a maternity swim top. It isn't that I have a problem with maternity togs, I just don't want to pay for them! She didn't say anything when we went swimming next.
Times have changed, as your mum pointed out. I bet none of the pictures of your mum when pregnant even had her in a tightfitted top. Even if she does find it weird she is probably trying to be OK with it and didn't really mean that she thought you were vulgar. Don't let it bother you!