Orthostatic Headache

My daughter (17) went to the ER last week because of the worst headache she's ever had.  She had one arm go numb, she was dizzy, nauseous and she had bad tunnel vision.  She parked her car and called for help.  In the ER, they took a blood culture, a urine sample, they did an EKG, a head CT and a spinal tap.  Everything came back normal.  They said that it was likely dehydration (she is an athlete), gave her a liter of fluid and sent her home.  Since then, she can not be sitting for more than an hour without getting a horrible headache.  The only thing to give her relief is to stand or better yet, to lay down.  Has anyone had these problems?  I've read that a CSF leak can cause this but she's had no trauma (concussion but back in April) and no surgeries.  The only thing she's had done to her back was the spinal tap.  BUT, it was a headache that brought us into the ER in the first place.  Although, she did say that these headaches are different than the one she had in the hospital in that there is no tunnel vision, no arm going numb or anything like that - just intense pain that is only relieved by laying down.  HELP!!  She's being recruited to play DII lacrosse so we need to get this figured out ASAP.  You can only STAND during class for SO LONG before you're just ... the weird kid who stands in the back of class!  She is laying down in the nurses office every day.  Any insight would be appreciated.