Hello,
Since the HIV test has come back negative after 8 mths, so chances of this rash being due to HIV is less. It can be due to folliculitis or acne keloidalais nucahe. It occurs when hairs on the back of the head and neck grow into the skin, become inflamed, and cause scar tissue. These ingrown hairs usually occur following a short haircut on the back of the head and nape of the neck.
Treatment of AKN includes suppression of the inflammation with antibiotics and steroid gels or intralesional steroid injections. So apply antibiotic cream like Neosporin or bactroban. If the symptoms persist then oral antibiotics and steroids may be needed. Please consult a dermatologist in that case.
Larger bumps of AKN present for a while or if not responding to treatments may need surgical removal. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Sounds like folliculitis related to cutting the neck hair; inflammation of hair follicles.
I would have this checked by your physician.