Alexis-
Your description of the kitty seems a little odd. I don't know if you are just being overly dramatic, or if something is wrong with the kitty. I don't think I would notice concave sides in a cat after a spay, nor do I think a vet would 'not tell' you if she was pregant...you seem a little uncertain about this vet, and I think I might watch things carefully, and go somewhere else, if you aren't getting answers to questions, or notice your cat looking suddenly different. (I don't think even a large BM would make the cat look concave- unless there was serious bloating going on). Just my opinion, but something seems off, and I don't know what it is.