What Causes Sores On The Face?

I am a 40 year old female. I have started getting these soars on my face that burn and sting, become festering and turns black inside leaving a crator hole after healing. I only get one at a time. It has happened 3 times since December of this year. On different areas of my face. Do you have any idea what it can be. My dermitologist has guessed at infected pimple to spider bite to he isn't sure but come back if it happens again. Help!! Each time it happens my face is scarred