I had Jazmine spayed at 6 months - she weighed around 3.75 pounds at that time and she did not bounce back after her spay as most dogs do especially large dogs - they seem to bounce back faster. I too was really worried about her I took 3 days off of work to stay home and keep her company - it took most of a week for her to get back to her old self. I think they are much like people - some of us seem to recover more quickly than others - she is my first little yorkie so I am very very attentive to her - as long as there is no sign of infection ( fever or redness around the incision) and her kidneys and bowls working which you say they are I wouldn't worry too much- I know it's hard not to
I called my vet 2 times the week after and he assured me that as long as there was no fever - throwing up and she was taking water she would heal in at her own pace and she did. Hope this helps
Marlene