She may be going through false pregnancy, My mouse went through it once but after a few days the swelling decreased. Or, Maybe her nipp;es are swellling because she is goin ginto heat and she may have worms or a parisite causeing her belly to swell. Maybe you should post about it in the pet health forum. Dr. Lee is awesome and will probably have An answer for you! Hes the best!
Good luck to you and your kitty.
P.s- Welcome to the forum! You'll meet alot of great ppl on here!