Was there a mole or anything there before or is this a new growth?
It's possible that you may just have a blocked pore that's just getting out of hand. It's a bit hard to tell how big this is, but it's likely not melanoma, which usually originates from pre-existing moles.
Given the speed at which you mentioned it is evolving, it may be a good idea to get it looked at by a dermatologist. Even if it's nothing, they may be able to pop it or put something on it to help it go away.