A single pus-filled pimple on the glans that bursts within a day is most commonly a minor local skin infection, such as folliculitis or irritation, and not necessarily a sexually transmitted disease (STD), especially if there was no recent unprotected sexual contact. Using someone else's undergarment can introduce bacteria or cause friction and may have contributed to the problem, so it is best avoided. Keep the area clean and dry, wash gently with mild soap and water, avoid squeezing or touching it, and wear loose, clean cotton underwear. Such lesions usually heal on their own in a few days. However, if you notice multiple sores, ulcers, persistent pain, discharge, fever, or if the lesion does not heal, consult a doctor to rule out STDs or other infections and get appropriate treatment.