Must I be worried about my eldery mother losing weight?

Systemic. Any sudden changes in weight could be an indication of systemic issues - weigh gain could be heart, fluid gain that is sudden, weight loss is worrisome is a sign body is breaking down quickly to make up energy need evaluation for cancer, or acute illness, parasites.
Depends. On why she is losing weight. A lot of elderly begin to lose weight...Decreased appetite, decreased metabolic feds. Some need to lose weight. If she is worrying you, then go see her doctor with her to be sure therenis not a medical problem causing the weight loss.