I have had this lump for two years and two days ago a constant pain developed. it's constant and it hurts to move my neck. what should i do.

See your doctor: You don't say where the lump is. From your brief description, this could be something called a thyroglossal duct cyst or a branchial cleft cyst. This could also be a lymph node. The first two are left-overs from the womb and can get infected. Lymph nodes are part of the immune system, but sometimes they get infected too. Or it could be something else. In any case, get it checked.