Passing Out!!

Hey,
Im 18 year old Female and i suffer with going to pass out. i dont PASS out but i go all faint and light headed i see stars and sometimes i see black and hear a like a sharp noise. i kind of get my self out of it. lasts like a couple of seconds and it happends when ever i go to stand up from sitting or laying down (it doesnt change anything if i go slower gettin up), when im in the shower (it doesnt matter if the shower is freezing cold or hot) , and when i go from walking inside to outside. When i went to the doctors he said i might be anemic but i had the blood tests and im not, so then he did my blood pressure a bunch of diffrent times he didnt see much. soo then he looked in my eyes for like 15 mins and was looking around he told me my brain is a bit slow?? he said when u go to stand up naturally ur blood wants to go to ur feet, but ur brain reacts and pumps the blood round he said my brain is a bit more slower and my blood gos towards my feet so then i panic or somthing like that. but i just want to know if anyone hs ANY idea of what this could be and have a better explaination for this?  

Thank You!