Hello,
There are two possibilities that I can think of.First is of folliculitis.Second is of acne keloidalis nucahe. Folliculitis is the inflammation of one or more hair follicles
grow into the skin, become inflamed, and cause scar tissue.
I would suggest that you get it evaluated from a dermatologist as treatment is different for both.
Hope it helps.Take care and pls do keep me posted on how you are doing or if you have any additional doubts.Kind regards