I fully suggest keeping track of when you get headaches, including what you were doing when you noticed it. Might keep track of daily activities as well.
The following are things that I know can trigger headaches. Obviously not all-inclusive:
1. Poor eyesight or other eye strain. Normal folks will get eye strain reading too-small text or staring at a computer screen. 2. Stress, anxiety, or depression: The muscles tense up, causing headaches. Massages help my mother, as do muscle relaxers and/or anti-anxiety medicine. 3. Sinus problems and/or allergies. 4. Dental problems. 5. Scents from various things or chemicals 6. Lack of sleep, sleep apnea, or other sleeping issues 7. High blood pressure (pharmacies and wal-marts often have machines to test, and I'd test often for a few weeks) 8. Food sensitivities 9. Neck strain (posture or medical). 10. Dehydration (easy to check) 11. Changes in weather. 12. Random viral or bacterial infections. 13. Low vitamin D. I personally had this happen, among other things. 14. Smoking or living with a smoker 15. Dry nasal cavities. You can use a saline nasal spray for this. I recommend disposables since they are sterile each time you open them (less chance of mold, etc). 16. Hormones. This might be more for women, especially with PMS and/or menopause. I'm not sure how it happens in men.