I don't know if you're still having trouble with your horse, I know it's been since last year...but just wanted to say, my guess is more than likely it is the saddle fitting. I could almost guarantee it if your horse is completely sound and is doing exactly what you describe. One of mine started this after a couple of rides in a new saddle I had. Sure enough, the bars were not wide enough and didn't fit at the shoulder. It more or less just pinched her. She would pull back and try to lay down and/or roll. Changed to a saddle with wider bars and after a few rides she completely quit. I know of numerous horses that had this problem and it was all the same, saddle not fitting correctly. Alot of these horses had the white shoulder scars that come from saddles not fitting. The scars in the horses' mind from a saddle will stay there forever though because they will not forget about a saddle that doesn't fit correctly!

I hope your horse is okay now!!