How long is it since you spayed her? Maybe it´s too soon and she hasn´t fully recovered yet? Though I´ve heard that many cats tend to become lazy after that, yours is too young to do so.
Usually, from my personal experience, she is supposed to be a bit lazy and down for at least a couple of months..afterwords she returns to normal I guess it´s due to hormone changes in her system.
It really depends on the cat. I have known cats that it changes nothing at all, and then others who have a complete 180. If you are overly worried I would have a chat to your vet. Me though, I wouldn´t be too stressed. Like us, some cats take a little longer to bounce after a surgery.