Help! Why would my horse do this!?!?

okay, so i have a 4 yr. old arabian. He was un-broke when i first got him in october, and of course was horrible for a couple of months. But then, he started to get better. We stopped using the snaffle, and went to a bit that was a LITTLE more severe..but nothing serious. I started to use spurs to get him off his forehand. And with the bit, and spur combo. he started to be really good! he was loping, and doing flying lead changes :D Unfortuanly, I cant show in split reins or that bit...i have to use rommel reins and an arabian bit. So at my lesson, we changed to rommel reins and the arab bit (it wasnt to severe, but straight across..and my horse has always been ridden ina broken bit) My trainer got on him first, and he was squirming a little, to get out of rein pressure but wasnt horrible. Then I get on him, he was fine to the left, i was loping him. And when I went to the right, he yanked his head to the inside..and when i tried to correct him he freaked out!!! before i knew it, he took off in a full gallop in our outdoor arena...I was using vocal commands, and was pulling as hard as i could to turn his head in..but i couldnt even get him to budge! He jumped the 3 1/2 ft. fence..i almost flew off, and then galloped up the feild going the road. I finally got him to stop, becasue he couldnt run as fast in the hay field. My trainer came running up, and I was fine...but a little shaky, becasue my horse has taken off but NEVER like this! we lunged him for 15 min. to the right, and I got back on him. All i did was ask him to trot and he took off again! This time luckily when he went to jump the fence...a girl at my barn, waved her hands at him to scare him away from it and he stopped. I cooled him down and walked him after that, so he didnt think that he could do that everytime. But why would my horse do this?!? It seemed like he had no intention on stopping! and why woould he react that badly to a differnt bit and one handed? Could someone please help! thank-you :D