Dizziness upon getting up?

Lately I've been spending alot of time in bed. It's not hot in my house. About 5 seconds after getting up I lose my sight for about 20 second, lose all feeling, feel weak, and all my limbs go numb. I'm 15 years old, I sleep about 8 hours during daylight hours, I eat approximately 750-1500 calories a day, I don't drink that much, I'm 5' 10'' and 130 lb. (I know I'm not very healthy). I also play video games for approximately 6 hours a day. (Once again I'm not very health conscious).