Hello,
It can be due to cysts, syringomas, hyperpigmented skin or infection of the skin. I suggest you to get an evaluation done from a dermatologist and get it evaluated.
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.