This can be due to oral melanotic macule. Oral melanotic macule is a benign, dark spot found on the lips or inside the mouth.
A biopsy has to be done and laser treatment is the main treatment option.
I sincerely suggest you to get it evaluated from a dermatologist. 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 dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.