I always break out on my chin with acne after going down on my girlfriend. and i know it is not herpes or canker sores. why might this be?

Bacteria: Bacteria from the vagina may be disrupting the normal skin bacteria on your face. Consider cleaning your face shortly after these encounters.

Could be caused: By local skin irritation triggering a breakout, or perhaps vaginal bacteria getting into your skin pores and causing pimple formation.

Rash : It is difficult to tell. But if it occurs consistently as described you may consider contact dermatitis.

ACNE: Acne is hormonal while herpes and canker sores are viral.It does not appear on the chin.

Bacteria: Bacteria from the vagina may be disrupting the normal skin bacteria on your face. Consider cleaning your face shortly after these encounters.