What would cause dizziness and shaking?

I am 20 years old, I have been in basically good health besides getting my tonsils removed when i was eight years old. I am extremely small, I am 5'3'' and weigh usually from around 89lbs-94lbs. The heaviest I've been is 100lbs even. I eat normally, 3 meals a day. I get check ups, and they always tell me I'm healthy. For a while now, I don't know how long exactly I have been getting random episodes o light headed-ness and dizziness. It's pretty random, sometimes I will be walking into my bathroom and get dizzy. When that happens my vision gets bright then goes black, and I try to sit down. Sometimes it goes away, sometimes I end up shaking rapidly and jerking around until it passes. If I don't sit down immediately sometimes I pass out and other times my knees give out and I shake and jerk around on the floor. I have no idea what it could be. It's been happening for a really long time. Since I was 14 maybe. I'm concerned now maybe because I am older and see the seriousness of it, or maybe because it has been happening much more often. When I was younger it would happen rarely and I always thought it was dehydration, or i hadn't eat or something of the sorts.