To get an accurate diagnosis of what it is, you really do need to go to the doctor- they are going to need to take a look at it in person and gather a bit more of your medical history to determine the cause. Skin conditions can be very hard to diagnosis, let alone tell you what they are online- it could be some sort of skin reaction, a long term skin condition, or something else- so go ahead and get into the doctor to have them take a look.