Lump on my forearm about two centimetres big was a soft lump is not soft anymore is quite painful when touching. What should I do?

Cyst. Probably a sebaceous cyst or fibroma that you have contused and caused discomfort. It should gradually improve with the pain disappearing and becoming soft again. If it does not let your physician have a ,look at it.
See a skin specialis. I suggest that you see a doctor (a dermatologist if possible) for an examination. This may be an inflamed epidermoid cyst and if so the treatment is simple, but an examination is required to be sure.