My mother-in-law has become increasingly bad tempered and aggressive over the past year, for no apparent reason. Any ideas why?

My mother in law, age 65, has become increasingly bad tempered and aggressive over the past year (for no apparent reason). She has no major health problems or pain. She gets out and about and enjoys her family - but increasingly, she is sharp with her family members, impatient, and often nasty. Her aggression is becoming increasingly unpleasant and out of character. I'm not sure whether it's just a bad habit that is growing because it has been left unchecked by doting relatives, or if it might be an underlying sign of something else. She suffers from occasional forgetfulness (eg misplaced spectacles) but has no other concerning symptoms.