I don't know why this keeps happening

Every once in awhile, I will get one or two small red bumps (about the size of the metal BB gun ammo or smaller) around the opening of my vagina (kinda towards the bottom closer to the gap between the vagina and anus.) they are a little irritating when I wipe, but they don't seem like they can be popped. ( I do shave down there but it doesn't seem to be an ingrown hair) It usually happens when I'm on my last days of my period. Is this because of friction... Or?