QH off on right circle, but lame on left side...????

For a few months now, my horse has been off on his circles to the right (at a trot), but looks as if he's lame on the left front. There is no swelling anywhere , nor any "hot" areas. I've tried circles in both regular grass and in a deep sand round pen. He does seem to be a little more noticeably off on the hard surface. He has had a little trouble in the past with absesses and thrush, which I thought it could be, but have also been told that it could be a muscle strain or torn ligament in the shoulder. What does it sound like it is? Is it likely that he has another absess and that is causing it? If so, how do you prevent absesses? But most importantly, what is causing my horse to be off in his circles to the right? Thanks in advance for your help.

Jonathan