My rabbit got really fat?

Ok so I don't know if I'm going crazy or what but I thought I submitted this question 2 already but I guess I didn't. Ok so my rabbit got really fat. She is 1 1/2 years old and Idk if rabbits just get fat in their older age or if she is pregnant. She has had babies before and that was the last time she was with a boy bunny and that was last year in June so that has been a full year. Also I understand that bunnies can hold sperm in their uterus for 6 months but as I said it has been a ful year. My mom says it could be a tumor and I don't think it is but I am scared cause I want my bunny to be ok. Also I don't want to get an X RAY becasue I once had a mouse that got a respiratory infection and my mom had to pay over $90 just to get the medicine and the check up so imagine how much it would cost for a bunny.Please help!!!:)