What Causes The Growth Of Lump In Anus?

Hi, so I have a few moles and freckles over my body but about 5 weeks ago a new mole started to grow right in my anus. It seemed to grow quite quickly over a couple of weeks and is about the size of a small green pea now(it s difficult to tell exactly how big). I have never noticed it before so I am fairly sure this is a new mole. It hasn t gotten much bigger over the last two weeks or so. It is a very dark brown, and I think it is quite evenly shaped but it is hard to tell. It has only gotten bigger (grown from nothing), it has not really changed shape. I may be being dramatic or overly cautious but I am worried it is melanoma; I read this on the nhs website, Melanomas usually appear as a dark, fast-growing spot where there was not one before, or a pre-existing mole that changes size, shape or colour and bleeds, itches or reddens. It hasn t changed colour and doesn t itch or bleed, but it did appear as a dark fast-growing spot. I am 16 years old, healthy and fit. I read moles increase in teenage years, so hopefully it is just a harmless mole. I guess the only way to know is to go to the doctors, but I wanted to as here first, in case I is really nothing to be getting so worried about. Thanks for the help and advice.