I had a time about 6 months ago where I had the same concerns as you. My symptoms started as stiffness/twitching/cramping. I started to get worried about things, started to research on the internet. Naturally, you initially find all the really terrible diseases that in any way match your symptoms. Then I started to lose weight and it seemed like I was losing muscle mass as well.

Once I thought I was losing muscle mass, I got really concerned. Then I started to feel weak, and had days I felt like I could barely climb the stairs.

Eventually, testing ruled out all the really nasty stuff. Over a period of weeks my strength came back, and medications controlled the twitching and cramping. Unfortunately, just when I thought everything was OK, the burning feet started........

The bottom line is that I am now convinced that I really never lost muscle mass, I was just losing weight because I was so worried about my condition and I wasn't eating right. Since then, I have taken measurements of my major muscles and recorded them. If I start to get concerned about muscle loss, I just measure and compare to my previous results.

I'm sure this is not exactly like your case. But, I do believe that any person who starts to closely examine their body will start to find things that they never noticed before or don't look right. And, that can kick off anxiety that just makes it worse.