Try not to worry (yeah, right!) this is most probably caused by a number of factors and will settle down given a few days.
The fact that she has been drinking a lot, she will be extra thirsty and have a dry, slightly sore throat after the operation, could be the only cause. That along with the stress and after affects of the drugs could be the cause.
It is also pretty common for the bladder to have been moved during the operation or even for a nerve to have been bruised or damaged. This may make it difficult for her to feel when she needs to go if she is asleep.
You should certainly phone your vet and talk to him/her about it and they may want to see her again.
Please let us know what happens.