Just wanted to update on my post about my cat Nala, who was spayed but still gets in-heat. My vet believes she could have pieces left behind that were not found by vet who spayed her. Recommends exploratory surgery. I've agreed to do this. It won't be cheap, but she is miserable and even seems to be in pain sometimes. Will post results of surgery.

She is lovely....wish I could help.

Here's the information from online:
Usually what we have is that there is some ovarian tissue outside the ovary itself, somewhere in the abdomen, that's not easily seen, that is still functioning ovarian tissue making the hormones and so the cat can come into heat, even after she's had her spay surgery. This is pretty rare but it can happen.Jul 22, 2011
Full article:
[petsbest.com]

I have heard of this - if they leave tissue behind apparently this can happen.

You need to see a second Vet. One who has heard of this problem.

Poor kitty.

I've never heard of that.
I'd probably get a second vet's opinion.