Really worried about my horse. Any suggestions please read!!?

I have a young four year old Thoroughbred gelding. Fit && healthy UTD on shots worming etc. Got his feet done last week etc. Great kind boy. Today I put his saddle on, did the girth up fairly loose, made sure it was wrinkled or anything. He was fine. He was standing in cross ties. I walked him a little up the isle to make sure it wasn't pinching(the girth) He stopped at the end of the isle. I grabbed a handful of gr, gave it to him. He chewed & swallowed. He was standing on all four feet. Suddenly his eyes rolled back slightly and he collapsed to the ground. I got him up and he was very disoriented. He reared up then tried kicking at another stall. He was so scared. Then he veered left and fell down again,leaning up against two stalls. I quickly screamed for help and undid his girth, talking calmly to him. He tried gettin up but struggled quite a bit and couldnt. The man and woman that own the horse barn came running along with a boarder who had been riding in the arena. We took off his saddle and got him up. He was shaking and scared. Still very confused. We disinfected a sc, let him stand quietly while we checked him over and talked to him, then walked him to be sure he was sound. Then I tacked him up and led him to the arena with the barn owners. I let him stretch at a walk and he was fine. Ten minutes later my trainer showed up for my lesson. I talked to her about it and let my horse stand. Then I urged him forward to show her he was okay. His whole left side was stiff. I walked him for a while to stretch then got off. He was sweaty after just walking and his eyes were SO confused. He never raced. I have had him a year and a half nothing like this has happened. Were going to keep a very close eye on him and if anything worsens were calling the vet. What couldthis be? What would cause this?