Hello dear, brief answer it can be sebaceous cyst, Detailed answer, I have gone through your question and I feel because of previous damage to the skin at that hairy area,there is development of sebaceous cyst which is because of obstruction of sebaceous duct of the hair follicle associated sebaceous gland.and that feels like you described in the history, I advise you to consult a surgeon or dermatologist to get it excised, another thing is that many times boil may get resolved but some firm painless small round soft tissue mass may remains that fade slowly, but that occurs exactly the same place of boil, so just see in your case,. Thank you