I'm 14, why do I feel bad when I get up?

I'm 14, why do I feel bad when I get up?

When I get up from a prone position (even if I was lying for about 15 minutes), my head starts to spin, darkens in my eyes, memory loss and convulsions. Sometimes I can fall to the floor or faint. All this lasts about 20 seconds, what could be the reason?

Read more