You probably got bitten by a brown recluse (fiddleback). Their venom is so toxic it practically eats skin if not treated carefully.Go see a doctor.
At this point it just looks like a large mosquito bite.I live in Southern California, and I do not know what bit me because it happened while I was asleep. Since there was never any pain (except after I itched it for an hour) it could've been a flock of mosquitoes for all I know.
Dude it leads to this;http://en.wikipedia.org/wiki/NecrosisGet it checked out.