Seizures?

Hey everyone i am new to this forum because i don't really know where else to go, Im 20 years old tall and pretty fit but ive been having seizure like symptoms happening to me when i stand up from laying down/sitting for a while. Its been happening since ive been like 15 years old. I can be laying down on the couch for a while and stand up to my feet and get the feeling of my vision going blury and feeling like im going to fall. Many time i will stand up and get blury but will fall down and have my body shake pretty bad sometimes like im having a seizure almost. These episodes only last like less than a minute tho. I can sometimes fall face front on my bed and shake for 10 seconds and at the same time be drooling/making weird noises from my body shaking kinda sounding like a horse. I just lay there waiting for it to be over but its like im slowly falling asleep/blacking out which i never did but i let all my air out and cant breath while its happening. Ive been to doctors for blood work, eeg test, heart monitor for a week, table tilt test, and some more tests. Everything came back normal with nothing was wrong. The doctors told me it was just from my blood pressure being low when standing quickly but why would i be shaking like this? After about a minute or two everything i normal and i can go to the gym or run a mile if i want. I dont feel any difference after it just worrys me that it will get worse. I noticed also after i stand up even slowly and start stretching the feeling kicks in and i fall down begin shaking, and am unable to control myself. Sometimes its just blurry vision and im fine to hold myself up, other times its just worse. Maybe someone has the same thing happen to them and would know how to stop it. thanks