What's Wrong?

My gelding has a problem that we can't figure out. When I ride, he's great on the flat, but when we start jumping he goes crazy. After a jump he starts bucking and galloping out of control. Today he bucked me off for the second lesson in a row. After that, a very experienced lady at my barn got on him and he tried to buck her off too, and she was just trotting him. It's just not him, and it has just started in the last month or so.
I've decided not to ride him until we get this figured out, because something's obviously hurting him. My trainer's calling the vet out tomorrow, but does anyone have an idea of what could be causing this? Thank you so much.

Have a vet check for gastric uclers. Look at his mouth and teeth. Check him all over. Spine, hips, neck. Check the saddle fit.

Gosh, it could be so many things.

Has he been eating all his food? Does he act differently at any other point or just after he jumps?
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Horse is in pain for any number of reasons. He is telling you the only way he can, he bucks when he is being worked hard.
Have a vet out to do a complete physical, blood panel and lameness exam.

I agree it could be NUMEROUS things. Get the vet to do a full check up.. Good luck :)

I think getting the vet out is definitely good, I've also found equine chiropractors to be very helpful in situations like that.