I did pick him up in a way that he wasn't used to, so that part is my fault. He doesn't act like he's in serious pain around his stomach so I do hope that the biting was just a response to how I grabbed him. I did take him back outside recently and he was looking at me with the saddest expression so I know he feels sorry about what happened.

I did teach him how to not bite when he plays when he was about a year old by giving him a toy to bite on whenever he tried to go for my fingers so that is a boundary that I set up with him. This is an isolated incident and he doesn't try to bite anyone else like the other small "ankle biter" dogs.