Hi yuyu,

I think 'weak' status has to do with health/energy, not with happiness. Is the energy- or the health bar very low? That could give you a clue on what to do.
Maybe she has been overly tired and/or ill for a long time?
If you have enough money, you could try a doctor-consult to find out what's wrong, and how to get her better again.
Good luck, hope she'll get better soon!