We went thru this with Ling. She was spayed back in January (this year). In July, she came back into full heat (I've bred cats, I know when they are in heat). I called the vet and she first told me that the cat probably had UTI that was causing the fake heat. I didn't believe it as none of my cats ever had UTI and I know the symptoms of it.
The other possibility would be cysts in the area where she was spayed. Ling went out of heat after a week. We figured everything was ok, maybe just a freak thing. But 2 weeks later, she again was "in heat". This time we got her scheduled for the vet to reopen her and find out why. It turned out that some cysts started growing on one side. Vet cleaned her out throughly and hopefully this will cure everything.
In all my years of owning cats and spaying/neutering this is the first time this ever occured to any of my cats.
I'd contact your vet and you may have to wind up with a 2nd surgery to find out why she's acting like she's in heat.