What Does A Bump On Mole Indicate?

Hi! I have a mole on my back that has a bump on it - it looks as if a pimple or something is growing underneath it and I m sure that s what it is, but I really don t want to risk not getting it examined as I ve read that skin cancer can spread in as little as three months. Please help me determine whether or not to visit my GP about this issue! Thanks so much for your help. :)