Is it skin cancer or is it a beauty spot with a pimple on it?

I have like a beauty spot that used to be flat on my skin and it about the size of a quater of a drinking straw. I think it might be cancer because its become a lump but it might just be that there is a pimple on it. I don't get much sun on my back so I don't think it is skin cancer but.. I don't know. I need some facts. Reassurence. Thanks