I Have Like A Pimple On My Right Buttcheek. Not

I have like a pimple on my right buttcheek. Not exactly a pimple but a bump thats a bit large but short, no clue what color it is since i cant see there. It ussualy comes, stays there for 2 days, bleeds a lot for 10 minutes and then bleeds a bit for at least 2 days, goes away and comes back 2 weeks if not a week or a day later. Im 15 and when I tell my mum she says its just a pimple at to put a bandaid on it. Doesnt work and is very painful to take off. Its impossible to convince my mum to get it checked out cuz she doesnt believe its anything. Its been happening for arround a year now and it is very annoying and painful. Please help