Can you see a doctor? It could be ringworm, cellulitis, a bite from a spider or something else, but the fact that it's getting bigger definitely merits getting medical care. If you can't get in to your regular doctor or a dermatologist, maybe try one of those urgent care places.
My suggestions would be the same as Annie's, but would only add that if it's warm to the touch, you are nauseous, have a fever, or get streaks coming from it, get to an urgent care or the ER.