First up, there's NO WAY it's a sexually transmitted disease. You can't get those without having sex.
(And masturbation doesn't count!) I used to get boils in that area when I was younger; they can be really painful but not serious (not that I'm a Dr, but they never were with me). If it doesn't clear up then you will need to have it lanced by a M.D. but nothing to be scared of. If it's causing you a lot of stress and / or pain take Luv's advice and tell your Mom to set up a appt., and make sure she knows it's just a boil or pimple . . . . you've haven't done anything for it to be something more.
Hope this helps. Take care.