Good evening caring mother,
The side effects of psychotropic medications (Lexapro being one of the SSRI's - selective serotonin re uptake inhibitor) vary from person to person. I personally am not aware of Lexapro causing weight gain/it is not to be ruled out though.
Individuals who are depressed and anxious may resort to eating comfort foods -processed sugars, refined carbs, chocolate etc in an unconscious attempt to naturally boost their serotonin levels. Has she monitored her eating habits? Metabolism also starts to slow about this time in one's life :(
Hormonal changes can lead to water retention, overeating, and mood changes ex: depression, irritability. Has she consulted w/ her OB?
Yaz is a birth control pill used for women who may experience premenstrual mood related symptoms.
As far as the GAD (generalized anxiety) there are natural ways (but she has to decide what she is willing to do and of course always consult with a physician about supplementation) and always consult with a naturopath or herbalist. As an advocate of natural methods here I have only listed some for informational purposes only!
Supplements that can have natural anti depressant properties: Amino Acid L-Tryptophan, S.A.M.E, Melatonin (to be take at night prior to bed ) - (which converts to L-tryptophan and that then converts to serotonin in the brain)
For anxiety: herbs - Lady's Slipper, Lemon Balm, (mineral Magnesium, ) Vitamns B-complex.
Another suggestion for anxiety that is natural is the use of self hypnotic tapes (cheaper than seeing a hypnotherapist in person but also a great option) Check out the CDs by Glenn Harrold (but there are also other good ones out there)
Other natural options- meditaion, walking/exercise, journaling, bibliotherapy, face to face counseling with a CBT (cognitive behavioral therapist)
There is a wonderful book out there titled The Natural Prozac.
Best of luck.