Why do I pass out when I go to stand up?

Okay so why is it that I pass out? I lay in bed or sit in my room and as soon as I go to get up ever thing go's blak and I get realy dizzy! I pass out I hav seen doc about it and they just say take time when u get up. Also a wehight doc to b.c I am 19 y/o and I'm only 80 lbs I'm on manny pills but nothing helps. I can't hold the Babis here b.c I'm scared I might pass out with them in my arms this is killing me! What can it be? How can I help it? WHAT DO I DO ???