Yesterday I rode Poco for about 45 minutes before I went to work and w/t/c just fine. I rode in his field and everything seemed fine. It was even extremely windy. Today was a TOTALLY different story. I got him saddle and did a little lunging, however, He would not can't. I thought maybe it was just the fact that he was on gravel so I didnt take it as a big deal. I triple check my saddle before I ride everytime. Well, I got on him and headed for the road. He was doing fine other than he was a little nervous which is usual. I turned him around and went the other direction because he was getting anxious over some dogs. Two school buses passed and he stepped up a little and looked, no biggy. Well I decided to take him down a little dirt rode where there is only one house. A truck pulled out the drive way with a trailer and then another truck following. Any other day this would have been nothing to him. He started spinning, bucking, rearing, the whole nine yards. NEVER in the 5 years that I have owned him has he ever done this. I got off. Got him calmed down and remounted. Same thing only this time he was taking me into the woods. I can't afford to stick it out and get thrown right now so I got off again because this is highly unusual for him. The only think I can think of is that fact that this is a new saddle. I walked him back to the barn and immediately grabbed my lunge line and whip. Took him into the field and started to lunge him (not out of anger). I done it just like any other day. He w/t and then canter, oh no! It was like asking for a miracle. Then all of a sudden he moved into a canter and bolted. I couldnt get him stopped. Finally I stopped him and headed him in the other direction, same thing. I had the gutty feeling that something was seriously wrong. I ran up to him and jerked my saddle off and threw it on the ground and asked him to move out. W/t/c just like any other day. I checked his back, no tenderness, checked his legs, stomach, chest, no tenderness or signs of soreness. The only thing I can really come down with is that the saddle just doesnt fit. He didnt mind me laying over his back or putting lots of pressure on it. He has had back problems before due to sliding down a hill but there was to soreness or tenderness there. Once the saddle was off he had this just like sigh of relief come over him. He dropped the pushy attitude and the trying to run me over and immediately became calmer and more tentive. I just wanted to see what you guys think because in a way I wanted to bring him home and give him a good work out but in the other sense I was petrified that it was something more serious. He has put on quite a good bit of weight and has filled out alot more but he wasnt acting excited undersaddle. He just wanted me off. He wasnt even headed home.