In Chinese medicine, acne on the chin:
Usually toxic bowels, hormone imbalance, too fatty diet, and impurities in the blood.
Hormones imbalances - check out liver, and look towards gonadals such as black cohosh for estrogen, wild yam for progesterone and saw palmetto or sarsasparilla for testosterone. For cleansing liver BP-X or LIV-A.
You are probably detoxing. If it carries on, you may want to start trying to see if anything is setting it off like dairy or wheat.