I have a lump on my upper forearm close to my elbow. it is fast growing and is increasingly painful. it is very solid and has started turning purple.

Skin Abscess: The description sounds like an abscess, or a localized infection. It likely requires an incision and drainage which usually cures it. This is a relatively simple bedside procedure. The body may be able to "wall it off" but there is a risk it will not. Can try warm compress, but advise to see your doctor soon since it is "fast growing".