Totally agree with Ghilly, especially with Christmas upon us.
If you are positive that she isn't sneaking off somewhere in the house and peeing without you being aware of it, I would contact the vet and let them know what is going on. Spaying, while a common, everyday procedure for our pets, is still a major abdominal surgery and shouldn't be taken lightly. If she is, indeed, having trouble peeing, then the vet should take a look at her. Keep in mind that tomorrow is Christmas Eve and the next day is Christmas, and many vet offices may be closed right through until Monday because of the holiday eating up the major part of the week. So if you are certain that she isn't peeing, call him NOW so that you can get her in there to be seen in case he does have to do something. Please let us know how she does.
Ghilly