I started noticing a irritating feeling on the inner cheek of my butt almost right before my vagina. It felt like an ingrown hair or like my panties had been rubbing on that specific spot too much. But when I went to go look down there to see what it was there was nothing there, no redness or bump or irritation. But when I felt it it hurt really bad and it felt like a small hard knot of some sort. What could this be?