Usually in these instances what is happening is that your kitty IS loosing body mass. She may have fluid in the abdomen or a tumor which makes her appear to be of normal weight, when in fact, she is seriously malnourished or suffering from a debilitating disease.
I would strongly suggest taking her into the vet so they can check her for hyperthyroid - which is very common in cats this age, diabetes and renal function. It's apparent that something is going on that is not normal, and she needs to be evaluated to determine what it is.
Please let me know if I can assist further.
Please Press ACCEPT if my answer helped solve your problem.
Lori McLaughlin
Your veterinarian should ALWAYS be consulted first and foremost when your pet's welfare is at stake.