What is this weird bump on the back of your head small about half size a pencil eraser and only hurts when you touch it been there for few months now?

It is possible that the bump on the back of your head could be a lipoma, which is a benign (non-cancerous) growth of fat cells. Lipomas are usually soft, movable, and painless, but they can sometimes become painful if they grow too large or press on a nerve. If you are concerned about the bump, it is a good idea to see a doctor to get a proper diagnosis and treatment. The doctor may recommend removing the lipoma if it is causing pain or discomfort.