Coming from a nurse, this could be a number of benign issues (one of the most common spots for boils is the butt, it's a warm and moist area, gross I know) that a simple round of antibiotics could help. I think having health anxiety makes it easy for us to jump to worst case scenario thinking. Also, looking on WebMd will only reinforce this fear....WebMD has a way of turning a simple cold into tuberculosis. I don't know if you do this, but I used to have the compulsion to look up every symptom, for reassurance, but then I realized it always left me feeling more anxious than before I started so I said "no more".
Definitely go to your primary care provider, even if it's your nightmare come true and it's skin cancer, the earlier it's detected the much better the outcome! But, just to reassure you, the "bad" kind of skin cancer (melanoma) almost always begins with a discolored mole.