I have constant headache and I don't know why

There's lots of reasons for headaches. Can't say this is true in your situation but sometimes doctors don't acknowledge some of the more obvious causes for things. For headaches it could be a lack of hydration or exercise, stress, diet, eye issues etc. So what I might consider is trying to figure out the why. So maybe addressing those few things I just mentioned might help you narrow it down.
I imagine you've tried this but run a search for the pressure point in your hand between your thumb and forefinger. When pressure is applied on both sides, and you breathe through it, headaches can often go away like right now. Your situation could be entirely different but I've never taken pills for a headache because that method works for me.
Good luck. Try to remember that's it's temporary. You WILL be back to normal.

The body is weird. Are you dehydrated? Blood pressure? Sinuses? Could it be a side effect of a medication if you are on any? Maybe COVID vaccine? There's a million things sadly that causes headaches. I used to get them so bad that I forgot what it felt like to not have one. Turns out it was my eyesight. A pair of glasses stopped it cold in its tracks

That's terrible. What do you suspect is the cause?I can only recommend home remedies. Try magnesium and scalp and shoulder massages. Reduce stress. Make sure you don't have dry eyes or vision issues. Check for dental issues. Could be anything, really.

Some experimenting might help.
- Does it get better if you drink lots of water?
- Try gargling with Listerine, maybe you have an infection in a tooth?
- If you cover your eyes for a few hours is it better? Sit in a dark room maybe.
- Could your issue be with a stiff neck? I have had that when I sleep funny. Try buying a new pillow, they make these cool ergonomic pillows that are really great.

Have you tried having aspirin which also are blood thinners?
Some headaches , like migraines, are caused by the blood cells expanding and if the blood is thinner it won't expand as easily and will help the headaches.

Do you eat a lot of sugar? Try eating saltier foods.. also how are your teeth? I often wonder if headaches and toothaches are related but idk

Perhaps

Try coffee and ibuprofen

Do you get enough sleep? Is your pillow comfortable enough?

What sort of tests have you underwent?