Have you seen a neurologist or reumytologist? Have you been diagnosed with anything?
If you have Myositis, working out too hard could cause more muscle damage that can NOT repair itself causing MORE weakness which will be doing more harm than good. Exercising within reason can benefit you but it has to be within reason as to not stress already stressed muscles. This disease is all about pacing yourself, and not over doing. That in itself, is hard to do, but it really is for the best! I havent mastered that part myself. I will usually overdue and then pay for days.
Anyway, thats my two cents ,for whatever its worth.
Wish you well,
take care, and God bless.