Unintentional and unexplained weight loss is definitely not good. There are many causes like worms in stool, depression, anorexia nervosa
, AIDS, cancers especially colon cancers, drug abuse, infections and loss of appetite. Loss of appetite can again be due to certain medications, drug abuse, depression, AIDS, acute and chronic infections, cancers and hypothyroidism. Either your mother does not eat well or there is increased metabolism as in hyperthyroid states, HIV, cancer etc.