Has she had a tilt? Any other symptoms?
One of my son's who was also 13 last summer had orthostatic hypotension. It was a bit disconcerting because some of his falls were pretty dramatic. Notably, he fell into our quartz counter top hitting his cheek bone and then was out cold on the floor for a second. Black eye as a result. Scared the poop out of me! For him, this happened most earlier in the day when he would not have eaten or drank enough. Especially bad if he slept in which he did a lot last summer (this year sports make that difficult and we're not having the issue as much because he gets up and eats). So, the doctor told him to get up and eat first thing in the am. He does this. And we work on his overall daily hydration. He plays soccer and has one to two long practices a day all week long and runs in the mornings for conditioning. So, now he has a water bottle with him at all times to try to stay on top of it.
Anyway, is there any pattern to when she is experiencing symptoms? My doctor led me to believe this was not uncommon for kids of this age. And suggested too that he stay aware and get up slowly from seated or laying positions.