What is this mystery sore?

Katie Asked: What is this mystery sore?It presented almost like acne, with a red sore about 2 centimeters in diameter with a rounded, pale white head. Instead of the head being bulbous, however, it seemed to be under the skin. Upon being squeezed, the head came out, and was not pus or a typical pustule head. It was a firm "sac-like" head. The remaining sore bled profusely and has a small hole where the head was removed.This was about two inches under and three inches to the right of the belly button, and there are no more sores around it, or anywhere else. It was not caused by

acne