I have a serious fever that comes and goes with a lot of pain. what could be causing this?

Fever and pain: how long have these symptoms been going on? how often do the fevers occur? where is the pain located? More specific information is needed for a doctor to be able to entertain a diagnosis. It could due to a common conditions such as the flu or to less commonly encountered conditions such as Adult Still's disease. Without more information your doctor can only speculate.