I don't blame you; I wouldn't pay for one either. I suspect your PT core work will hit your back, too.

I think your theory about imbalance is probably spot on. One can be very, very strong but if you have a hypermobile joint on one side (or a sticky joint, like your SI), or something tweaky with your back, or whatever, it's easy to get off kilter over time. Hang in there...I know it's frustrating!!!