Pinning ears

I hope to post a video of this behavior soon. Yesterday I decided to ride Boomer a little without longing him first. He has taken to pinning his ears when I tighten his girth, so I go slowly and tighten and loosen it until he stands quietly. He also pins his ears when I mount. I generally have John hold the off side stirrup to keep the saddle centered and not pulling on his withers. Once I didn't have him held the stirrup and it caused him to pin his ears. Now, even with John's help, he pins his ears when I mount. Yesterday he was doing very well when we started. We walked around both ways but when I asked for a trot he pinned his ears. He continued this behavior no matter how I asked him- just voice, body posture, legs or a combination. I tried circling around John like we were longing and John asked him to trot, he did trot at first but then pinned his ears and would stop suddenly, tossing and snaking his head towards me to the left. Concerned that he was in pain, I took things down to a walk and we just cruised around. He was happy, comfortable, willing, and enjoyed himself. We walked all around the paddock and then went throught the gate (great!) and walked around the rest of the acre! He walked briskly, had pricked ears, and breathed heavily- not in the scared snorty way, but in that fun loving way. He was great over all. I got off and left him tacked up and longed him at a walk and trot to see if he was having pain. He tossed his head the first time I asked for the trot, but got over it quickly and was perfect! After I untacked him I palpated him back and spine, withers, neck, and rump. He showed no signs of pain or sensitivity. I tried my saddle on him before without a pad and it looks like it fits him perfectly. I do not think he is in pain. I think it may be a few things. He may have been upset because I threw off his routine by not longing first, he may be cold backed and need to be longed first, or he was testing me. Of course, it poured all last night and today so riding probably isn't a great idea today since we are trying to figure out a problem. Maybe tomorrow I will try longing him to get him warmed up first and then try trotting again.