Why do I feel dizzy when getting up?

Anonymous
Whenever I lay on my bed, say for 10-20 mins., I feel as if I'm gonna faint after getting up and standing. I've been suffering this for 2 or 3 years. I'm 15, 121lbs, 5'7" in height. I usually don't exercise much, but I recently started. I sometimes eat junk food. But people tell me that I'm a fit person. I am thin.
Why do I feel dizzy when getting up?
2 Opinion