What causes low blood pressure?!

What causes low blood pressure?

Whenever I am laying down and stand up too fast I start to black out. I get lightheaded and feel my heart speed up a little and sometimes start to go blind for a few seconds.. It doesn't really happen if I don't suddenly jump up but every time I do I get dizzy. This has been going on for over a year. I know I have serious anxiety issues so I'm hoping thats the cause. What else could it possibly be caused by? Thanks.

2 weeks ago
I'm 16 years old and have had my blood pressure taken for a few hours when I was in the hospital for a panic attack lol. I think if it was low enough they would have told me.

i have a similar problem, except mine is caused by anorexia. i was told that if i'm feeling faint, i should sit down immediately (lay down if you can) and elevate your feet slightly. I was told that it can happen because of dehydration issues also. Just try to get up slowly and get yourself to the doctor to make sure its nothing serious.