Hello, I can understand your concern for small painful skin tag growing on buttock and its propensity to be cancerous. After reading your query it appears to be related to be simple skin tag which we call as Achrochordon which is large tag. Let me reassure you fully that these skin tags never turns cancerous and the only problem is to get infected. These kind of tags are known to get infected frequently because the large size and repeated friction along with inflammation. If I were your treating doctor then would have prescribed you an antibiotic such as Amoxycillin or cefixime along with pain killer in proper dose to reduce the inflammation. I would have recommended excising the tag at the earliest. You can meet a dermatologist for further guidance. All the best.