What's myopothy and if is serious to have for three weeks?

It depends: Myopathy simply means "muscle disease", and that can include a wide range of problems, but usually refers to a problem within the muscle itself. I would recommend that you see a specialist to clarify and/or confirm your diagnosis and discuss with you your condition in more detail.

Myopathy: Myopathy is a disorder of muscle. There are many causes from inflammation, infection, metabolic disorders and degenerative disorders. I would discuss this with your doctor who may refer you to a neurologist or rheumatologist. Since you have only had myopathy for such a short time, you may be experiencing an inflammatory process, for which there are several treatments.