What Causes Dizziness, Blank Vision, Knee Weakness And Numbness?

Hello, my name is Heather I am 17 years old. I frequently get head rushes, my vision becomes blank, as in everything turns black. My knees get weak, & sometimes I even fall over...Yesterday, I was at a friends house. It was late I went to the bathroom, & suddenly got the urge to vomit...I started to get dizzy & had a painful ache in my stomach..It felt like my stomach was in knots.. My friend came in because I had started the shower hoping it would make me feel better. As I opened the door, I started to shake uncontrollably, I fell to the floor and laid down. My face got really pale. I looked in the mirror n my vision was blurry, I could hardly stand. As I laid in the bath tub my legs went a little numb. I still felt dizzy for around 5 to 10 minutes. Cannot you help me? I'm worried of what it could be?