Crusty bump on vulva. . .?

yesterday i noticed a what appeared to be a pimple or small blister (about the size of the tip of a pencil). it was red and itchy and painful when something rubbed against it. i put prescription strength acne medication on it hoping it was just acne. well when i woke up this morning it is still red and now it is crusty, but it is not itchy anymore. is it possible the acne medication made it crust? or could this be something more serious like genital ? i've never had this happen to me before. i am a 20 years old and ually active in a monogamous relationship with my boyfriend for the past 3 years. it would have been 3 years and 1 month since my last exposure to any std's. i have an appointment with my gynocologist on may 4th (the soonest i could get in) but i'd like some peace of mind until then. thanks.