We know nothing about your age, gender, history, and general health now, and you need a complete physical. So see a doctor. You could have any number of problems that range from mild to serious. It could be rare, it could be slow and degenerative, it could be something hard to diagnose at first, and there are several that are the 'rule out' type. It could be from a stroke you didn't know about, or just a pinched nerve. Or MS, MD, ALS, Parkinson's... anything.
And if MDs can't help, see a chiropractor.
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