What could be the cause of frequent headaches? Most are small but bi-weekly, or so, I get a major migraine.

Headache. Your headaches coudl be coming from tension, eye strain, tooth grinding, dehydration, lack of sleep/poor sleep, skipping meals, stress, sleep apnea/snoring, or chronic sinus problems.
No one konws. Sounds like typical migraine. Causes can be certain foods, lack of sleep, certain sounds or even just the right light reflections. Treatment based upon prevention and avoidance of rebound medication dosing.