Hi Sunny, welcome to OCF.

Anytime a persons immune system is activated for any reason you can have the symptoms you are having, either from cancer, infection etc. The triggers that activate your immune system also inhibit digestion and absorption, while increasing your bodies need for amino acids, specifically the amino acid leucine. Whatever the body isn't getting from nutrition it will kick into a catabolic state and begin to breakdown it's own "stores" to throw at the immune response. Generally it will start by breaking down lean muscle mass which is the easiest, fastest source of amino acids.

To help your symptoms and the way you feel address your diet while your physicians work on finding the cause. Up the amount of protein in your diet using a supplements, Whey is the fastest absorbing form of protein if you don't have dietary restrictions (vegetarian/vegan), Branched Chain Amino Acid supplements, and addressing your vitamin deficiencies through food which the body absorbs easier than isolate vitamin supplements. (fish oil is a great source of vitamin d and Omega 3's both of which will help your body metabolically, vitamin d being a prohormone sterol which is a precurser to testosterone and omega 3's helping digestion due to it's effect on insulin resistance, only downside typically being heavy metal content, Red Star T-6635+ nutritional yeast is a fantastic source of B12 and is vegan friendly, Folic is found in spinach which I blend in my shakes).

Other than that keep looking for answers and good luck.

Eric