i'm worried

okay so i haven't had sex and i have this HUGE blister on the top inside of my thighs near my vagina and it went down and now it swelled back up again. its not white or slimey with puss and there's only one. is it because my thighs rub together or something?? UPDATE: i looked at it closely with a flash light and it's a purpleish/redish blood blister maybe?