Oncologist... Discuss your pain with your oncologist. Be honest with what symptoms you are having so they are in the best position to determine a course of treatment. Open communication is important. If the pain is significant, the oncologist may wish to involve a pain specialist to help manage medication choices.
Call your physician. Pain in cancer can have many causes. If you have had surgery for cancer, it can be related to the surgery itself. Some cancers can irritate local nerves. Some can cause blockage or the outflow of various organs. Obviously, it all depends on the type of cancer and what has happened during its course.
Pain meds. Your oncologist should be able to prescribe pain killers geared specifically towards the cancer.