Meg it sounds like it could possibly be a hernia. I know when we have them at the shelter sometimes they seem small and other times seem big. It just depends on how much is falling through the tear/hole at the moment.

Little Freddy had a hernia and sometimes it felt big but if you pushed on it it would almost disappear for awhile. According to the vet as long as it stays soft they can hold off on surgery for a bit but if it gets hard its very bad.

Its just a thought but you might want to keep an eye on it when she visits again.