Major blackouts when I get up to walk, what is it ?

Well I lay down, or sit down, and I go to get up and walk somewhere, and my head feels light headed and I begin to blackout. My vision begins to blur, And everything fades to black. I drink lots of water, eat to fulfill my hunger, sleep a good 8-10 hours. I am 15 currently, female. I am not sick, I am totally healthy. Well no sickness, not that i know of... And right when i black out i feel like my bodies shutting down. But I regain stability and everything goes normal. The blackouts last about a good 15 seconds. I had an incident where I got up slowly like always because getting up fast makes it worst, And I went to walk into my hallway, and as I began to walk, My vision began to blacken, and slowly my body began to shut down in those few seconds, My mind shut off, and I fell to the ground and hit my left eyebrow on my vacum, wall, and floor, Once I fell and my face was bleeding, My senses came back sorta, but I couldn't feel the pain of the gash on my eyebrow gushing blood. As I got up I felt very dizzy of course considering i had just knocked my head on some stuff. But I really need help, I wanna know whats wrong with me. Please help me...... iDK if i need to check up on my brain or whatever.