Question about my thoroughbred

Question
I am 19 years old, I have owed and barrel raced horses all my life. For the last five years I have been barrel racing an off the track thoroughbred. He is 11 years old and well he is always pretty hipper when it comes to running but aint really a problem. Well here lately he has really started jumping around and spinning when entering the ring worse than ever. When brushing him to ride tonight I noticed he kind of ducked when I went to brush over his back with the curry comb. I tried to push down on him to see how much of a reaction I would get,but didn't recieve any. I don't know if I just didn't push hard enough or not. Well I went on to ride him tonight and he started jumping around again like crazy. It is all just making me wonder if maybe it is a back related problem. It is really starting to make me wonder. From you knowledge what do you think could be causing this problem?

Answer
I would have to see his reaction, try putting pressure on him and saddle and see if you get any reactions.  Barrel racing horses get conditioning to respond that way from years of being run fast and hard.  I would say it is an learned response rather then pain, but not sure without seeing him.  

Hope this helps,

Rick