What Causes Red Sore On The Leg?

A year ago I noticed a deep red sore on my leg, the size of a dime, with a white circle in the middle. It healed. Six months later another one appeared, and went away. Now my legs are covered with them. This is not a rash, as they pop up here and there. They last for several weeks and then dry up, but leave a scar. My doctor says not to worry about them, but I am.