Hello! Actually, this happened to my dog when she was spayed several years ago. In her case, she had a bladder infection. This is not uncommon after spaying. I have heard other possibilities, too, which are as follows:
Hernia developed after surgery
Pressure on the bladder from surgery
Some dogs just take a little extra time to recover from a surgery such as that. You also have to think of the "what if you were in her position." Either way, definitely contact your vet.