how old is your mom? if she's in her 40s-50s like others have said, menopause could be the cause, have her consult her gynae on treatment.

is she overweight? have skin issues? many women tend to go into depression over their physical status.

how is her marriage life? alot of women find that they feel neglected as they get older and get less attention from their husbands who are busy working.

is she working? she could have work related stress which could be causing her depression.

you said she told you that she's suffering, did she mention why? does she know what's causing the pain?

too few details to give a proper diagnosis and we're not doctors or psychologists so even given the details we may not give a accurate diagnosis.

important that you not show her your stress as that would further stress her.

if its because her health is getting worse, she should try to eat a good diet and get some exercise. its expected that her health will go downhill somewhat with age, as she comes to terms with that her stress and depression over not being as healthy as she once was should lessen.