Weird rash on the back of my thigh?

its been there for about a month. at first i thought it was just a pimple, but when i popped it, it got bigger and more sores appeared. it hurts and scabs over when i pick at it. its only on my right leg.. i don't think it could be an std or anything, cause i'm a virgin, but i have done oral stuff... could it be from sitting on dirty surfaces maybe? what is it?