Is it hard? If it was then it could be some kind of growth. Or if you can get him to sit still a bit, can you find a little spot that seems like a centre? Like a bite or sting mark? I'd probably call and make an appointment anyway, if it starts to get better you can always cancel it. And considering you'd have to wait all weekend otherwise it'd probably be better to be safe than sorry.

hope the little guy is ok!