What Causes Frequent And Long-lasting Headaches?

Hi, i have been having sever headaches daily know for the past 3 weeks, i had this before. When i did we took action and i went to get blood drawn and an MRI done. They thought i had what was called Chairi Malformation? but my brain wasnt tilted down enough to consider it that. So i went to Childrens Hospital on June 9th 2010 and they monitored the pressure of my brain for two days and found nothing. they went away for a while but now iv been getting them bad again. im afraid something is wrong. please help