Any educated guesses out there?

This morning we checked Flash's mystery injury.  It was very oozy and tender.  For the first time, he really didn't like me poking around in that area.  He's still perky and eating and otherwise doing fine.  But it is worrisome nonetheless.  Brett called the equine hospital and made an appointment for tomorrow.  He wanted to take Flash down today but it's a $250 emergency fee on Sundays so he is going to wait until tomorrow.  It almost seems like an abscess and we wonder if it is related to the dental work he had done.  On the other hand, there isn't any fever, there wasn't pain (until today), and it started with a break in the skin followed by swelling and then oozing.  I've only dealt with hoof abscesses before and they seem to build with pain and swelling first, then a big stinky messy eruption and blessed relief.  We'll know what the deal is tomorrow but, in the meantime, I'm wondering if any of you have ideas.  Here's a picture -- it's not great.  Flash didn't like the camera - especially when the flash went off in the barn.  I ended up standing outside and getting the picture while he was sticking his head outside.