Hello,
From the symptoms it can be due to labial melantoic macule. It is a small, sharply circumscribed, pigmented macule surrounded by normal-appearing skin.
I would recommend a dermatologist’s consultation for this.
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.