Maybe she is just taking a little more time to get back to herself? Some only take a day or two, other take a bit longer. It is a major surgery. if the vet says no infection then I wouldn't worry too much about it. My Uni got spayed last month and she was back to normal and humping her bed again in about 3 days. Yes she still humps!
She have a voracious appetite now though! Other than that she hasn't changed.