Depends. Many possibilities. How old r u? What is your sex? Other symptoms and signs need to be examined. See your doctor who may consider disease like temporal arthritis.
See physician. Most headaches not serious. However, first your doctor will determine what type of headache you have. Headaches have many causes I suggest you see your physician and discuss a plan to determine the cause. Your doctor might order imaging of brain and optic nerve exam to rule out serious conditions.