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Sounds like it might be a spider / insect bite of some kind, but that's just a guess. I would carefully wash it, apply an antibiotic cream and then cover it with a band-aid. If it doesn't look better in a few days, see a dermatologist. There's literally 100's of things that can go wrong with your skin, most fairly benign.
Monitor it though - if it starts to swell, become painful or you see red streaks emanating from it, time to go by the emergency room to get it checked by an MD.