...really fast.? I have no idea of the shape or size, other than it either didn't hang on for very long and or it wasn't heavy.

I live in MIssissippi (sorry bout the location, but the regional thing is important) and we've been gone for like 2 days, and it was dark outside and I opened my mailbox (I know, I even thought it was stupid before I did it, but nothing has bitten me, especially in a mailbox, in like years). So I opened it, stepped back to kind of peer in and then reached, and was bitten really fast.

It hurt really bad, there's no stinger in it, it is just a single, red tiny mark. The area around went really white, with the red bite in the center. It bit me on my right ring finger, and after about 10 minutes had swollen the area for about an inch diameter (including the finger next to it and the palm of my hand) around the bite (swollen and red.)

It's not a wasp or bee, or probably a spider because I was bitten really fast.

Any ideas/guesses as to what it was? At what point should I see a doctor? What medicine should I take (i've taken benadryl)?