Is the cat in heat?

Yesterday I was getting ready for the December Birthdays birthday party (thanks, Nana!) and after I had showered and dressed, I put my robe on my bed. Then I ran around the house doing a bunch of other stuff. About half an hour later, I walked back in my room and saw Sophie on my bed straddling my robe with her butt pointed toward the ceiling and the neck cuff of my robe stuffed in her mouth. She turned and looked at me and she didn't even bother to drop the robe out of her mouth. We stared at each other for a few seconds, then I felt I should leave the two of them alone so I turned around and walked out.

The cat is supposed to be spayed, which means she's not supposed to go into heat. And she hasn't been screaming or gyrating or doing all the other things female cats do when they are in heat. If she does, I don't know what I'll do---there is nothing more horrifying than a cat in heat. All I know is if I walk in and find her standing over Grover biting his nose, there's going to be trouble.