First thing you need to do is go to your physician and have a battery of tests run to make sure there are no internal, serious problems such as blockages causing lack of oxygen or worse. Not knowing you, no one here could possibly guess why. lisajane87 is right, a lack of water can cause headaches. Could be stress causing tension headaches. You really should see your doctor.
monosodium glutamate (MSG), nitrates, and possibly chocolate, caffeine and alchohol. Migraines are triggered by these things. You would be awesomely surprised at how many things have MSG in them now. All lunchmeat, ham, bacon, hot dogs and sausage have nitrates.
You may also be sensitive to artificial sweeteners such as aspartame, sucralose and saccharin generally found in gum, mints, medicines and low sugar drinks and candies.
My husband suffered debilating migraines for years. When his neurologist gave him a list of foods and food additives to avoid, he never got another migraine. If he did, it was light or stress induced... but still nothing like what he used to get.
See a neurologist if you can, you won't regret it. If you can't go, try the migraine-trigger elimination diet and introduce one thing at a time to see what is giving you the headaches. My husband is even affected by nuts, fish, red grape skins and potato skins (all these things, while generally beneficial, expand the lining in the brain a little bit).
If you have any questions, let me know. I've become a bit of an expert on migraine diets (and I've had to adjust my grocery shopping and cooking) over the past 15 years.
It could be allergies and sinus problems. I am allergic to mold, for example, and I moved a piece of furniture into my room that had been stored in a cellar. I didn't know it had grown mold on it and I was sleeping in the same room with it. Every day I was waking up with a headache.
Examine your lifestyle. Do you live with pets and plants? If there is an animal that sleeps with you can you banish it? Do you live in a dusty environment? Is there mold anywhere nearby? If there is mold around, can you kill it by spraying it with Tilex? Do you sleep with a feather pillow? Do you wash your bedding every week? Is there carpet in your room? Is it vacuumed with a Hepa filter vacuum cleaner every other day? Upper sinus blockage is just a possibility, but try cleaning up your environment and see if it helps.
Its not uncommon for hormones to cause migraines too.
I would see your doc though!
bad eyes, or you are not drinking enough water.