If you haven't hit or jammed it on anything, then it could be an infection, bite or something I'm not familiar with. A quick trip to the doctor's is your best bet, you'll need some antibiotics most likely. Couldn't hurt to go.
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Not that I can remember no...
i had swelling like that on my leg but of course mine was probably worse because it was on my leg but still that looks very bad i would get that checked out A.S.A.P