Good she is spayed, that rules out one serious problem!
It is also good her gums are pink.
Lethargy, weakness, and not eating can be signs of many things: internal organ disease, infection, something she ate. If I could, I would start with a good physical exam, taking her temp, feeling internal organs, pulses, listening to her heart, etc.
Until you can get her checked out by your veterinarian, I would try to keep her hydrated by offering water, broth or Pedialyte. If she seems nauseous you can give Pepcid, famotidine
, at 10 mg to 15 mg twice daily.
Let me know what else I can answer or help with.