My dear, there are some things our mothers do not tell us and this is one of them:
You can get acne on your vagina! If it's a white head then chances are it really is just a spot in an unfortunate place. I've had them on/off for the past eight years and they're an absolute b*tch but they do no harm. If it stings really badly, put sudo cream or some kind of nappy cream on it, it really helps!