Is it swelling at all? It looks like some sort of localized allergic/inflammatory reaction to something. Try to think of anything off you've touched/been exposed to.
If its itching and swelling put some benadryl cream/cortisone cream on it and see if it goes away.
It's probably that if you just saw it on that area only, if its all over your body it could still be that but we would probably need a blood test in that case.
Obligatory: First year med student I dont know anything, etc etc etc