I have had a small pea size hard bump on the back of my head for years now all of a sudden it hurts to touch?

Please seek help: It is difficult to comment without having look at this swelling. If you haven't injured yourself, this swelling could mean an abscess or a cyst. Please visit your doctor for an examination and further investigation

Cyst: The most common cysts are either sebaceous or lipomas. Sometimes they are contused by being struck with something as simple as a brush or comb. This should settle down and revert to what you had before

Sebaceous cyst?: As per the previous answers both sebaceous cysts and lipomas are common and can become inflamed/painful but impossible to give a definite answer without examination. Your doctor if not sure after examination may need to refer you for scans to clarify.