My kitty was spayed one month ago this weekend, and I have noticed the same thing on her tummy. The lower end of her incision site has a lump under it. It seems to have gone away somewhat, but I am curious as to what it's all about, too. My vet took maternity leave a week or so after Maggie's spaying so I don't really want to call and bother her since it doesn't seem to be an issue for my cat. I've had male cats previously so this is new to me.