You are having a blood pressure crash. That happens to my husband also. He has hereditary injection/blood/injury "phobia", which is not a real phobia but a type of shock response. He has lost consciousness and injured himself by falling and smashing his head when he has seen others injured and bleeding.

He can also hop in and act to care for injured people and even preform simple proceedures, look right at people with horrible injuries, and manage it fine until he gets to a safe plac, and then the syptomes hit. Theses are the blindness, throwing up, shaking, collapse, etc that you describe.

I suggest you let the doctors know ahead of time, have water on hand in the room and make sure that you DO NOT GET UP, let your dad or the doctor give you the water lying down pr nearly so until you feel your head clear. Think of this as a head rush, it's kind of the same mechanism. Do whatever makes you feel calm and reasonably relaxed before you go in: this isn't going to be dangerous. Do not let yourself be hurried, and keep things around you as calm as possible. HUGGGGSSSSS!