I guess that's the downside of practicing technique: you risk repetitive stress injury. I've dealt with RSI at various points in my life, generally during periods of high stress and excessive typing. The solution, if you catch it early, is straightforward: stop! And, just as important, get more sleep. I think it's partially immune related, so stress can be a bugger. I started getting some twinges a week ago, then last night my left thumb began hurting. I don't need to practice eight hours a day, so it was easy to decide to just stop immediately, and try and get some sleep. It'd also be helpful to try and get the mechanics sorted out, so that the fingers are always in natural positions, and not moving long distances, but for the left thumb, I find that hard. To access the low keys, D through Bb, I usually pivot at the base of my thumb, and have it bent backwards. Maybe I can do something different, or maybe my muscles and joints can get used to it.
I also found the Health and Wellness forum on the IDRS site. Musicians have a lot of medical problems. In the States, it's particularly bad, since many pros work gig to gig, and have no insurance coverage, in addition to losing their source of income due to the disease. One oboist, practicing for job auditions, seemed to have worked herself into a variety of mechanical problems. And also, to me it seemed, maybe a bit of target panic/focal dystonia, terrifying disorder which seems to be caused by wearing out the mental circuits for a task through overuse. It can be permanent, resolvable only by retraining to do the task in a completely different manner (eg switching from left-handed to right-handed), or giving up on the task entirely. Overtraining can be devastating in many ways, something to think about before practicing all night.