I have had the same intermittently for the past 12 years.
There are times I have stood and collapsed to the floor, seeing an indentation like a triangle above my knee. If this happens and I do anything requiring effort in the following days I will typically lose the strength of certain muscle functions. I used to get them back with effort, but I haven't seemed to lately.
Other times I have a sort of crisis where my heart function seems to tell me I am near death. Lying down immediately, being as cold as possible, and eating a (salty) cracker at a time until an entire sleeve is finished followed by slowly sipping water seems to help in that situation. That is usually followed by a few days of severe muscle wasting.
I have also just lost muscle slowly over time, but in a way that other sedentary people don't seem to; you can, for example, see every rib in my chest now.
Over the years, they have found my CPK/CK to be over 2,000 a few times (other times normal), but never at a time of crisis (because then I am too ill to go anywhere at all!) You might asked to get that checked if it hasn't been already?
I hope in some way this reassures you. When this first started I was told I must have a neuromuscular degenerative disorder and it was terrifying, but over a decade later with no real diagnosis/explanation or treatment and it has only been in the past year that my problems turned the corner towards doom. The odds are that you don't have something quickly catastrophic. I have been limited by a lack of money for healthcare (I have thought at times during crises that a funeral would be cheaper than a hospital visit!), but it sounds like you have doctors looking for answers. So be hopeful knowing many of us have experienced similar and do not have a dread diagnosis. During my remission I was actually able to add some muscle back. That must mean something good!