Sounds exactly like anaphylactic shock bro, a very scary event. It has happened to me several times with my injections. It also happened to me once at the doctor's office so I was explained exactly what was going on.
I got a cortisone shot into the bone on my wrist and it happened. I thought I was going to die as I began to fade to black. Luckily there was a doctor and nurse on hand there and they explained to me in detail exactly what was happening to me. The doctor said it happens to different people at different times but basically when some people get an injection, their body triggers a reaction and they go into anaphylactic shock where the opposite of the fight or flight response occurs. Instead of all of your blood vessels constricting in a fight or flight response, all of your blood vessels dilate at once and your blood pressure drops extremely low very fast. This is why you go dead weak, feel like puking, and start to pass out. According to Wikipedia, a small percentage of people die from this event.