My block heads have done that once or twice. They know they hurt me (on accident) and don't leave my side for hours after it happens. If it was broken, it would have bled, or started swelling immediately, or you'll wake up with a black eye. (Only speaking from experience thanks to a drunken swimming incident).
Considering how easily my small dogs can hurt me, I can only imagine the damage big ones can do. The older of my two dogs once bit through the nail on my pointer finger (my fault, we were rough housing). She was so upset over it, I think it hurt her worse than it did me.