Thanks for explaining the problem in more detail. Interesting, and slightly alarming, that one can strain a muscle like that but, as I said above, I've had some quite interesting experiences where the body seemed to be straining itself to get straightened out (while I just observed...), and other experiences where I moved and shook. One of my monastic teachers said that he'd seen people shake so much they whacked their head on the ground. (This was back in Bangladesh - where he's from - on Mahasi-style retreats). I'm guessing that your retreat was a Goenka one? I've only been on one Goenka retreat, back in 2007, but that was my first long retreat, and the first time I really had this sort of thing happen.
Hmm, in conclusion, I don't seem to be offering anything useful, but I would try to discuss it with your teachers, who may well have seen similar problems.