I believe the answer is that she wants attention from you. Play with her instead of letting her playing alone. Be sure to give her attention when she comes to you for it, and make sure the dog and the cat are getting along. Windows and high places to hide are also good things to entertain a cat. I also suggest you make play a routine so your cat doesn't keep getting bored. As for the yowling, it could be leftovers from her hormones as her levels drop; or, it could simply be her way of getting you to pay more attention to her. My cats, both males and neutered, will call occasionally to each other by yowling from another room. Then, when the other one comes, they pounce and the wrestling begins. She may also be calling to another cat that she can smell outside. You should check for strays just in case that is the case.