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.
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.
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