Headaches not affected by painkillers?

I've had a pounding headache for the past two days, and I've tried a variety of painkillers, paracetamol, ibuprofen etc and none have done anything to help. I have also been taking hayfever tablets in case that was the problem. I dont drink coffee/tea so its not caffeine related, I'm not stressed at all, and I don't smoke. Iv'e been making sure that I drink 2 litres of water a day, and stay out of the sun. I have found that if I lie down or sit down, the headache mainly goes away, but whenever I stand up again, bend down or walk around (especially going up/down stairs) and even moving my head causes it to start up again til its really painful. It also sometimes hurts behind my eyes aswel. Does anyone know what could be causing these headaches?