This happened to Ling - she was spayed at 8 months old, about 15 - 16 months later, she showed all the signs of "in heat". The vet wound up having to reopen her and found 2 cysts growing in the area where the ovaries are. Once they were removed, the "heat" was over.
Mentioned to your vet about the possibility of a cysts in that area.
BTW you are the 4th (including me) person I know that females had this problem. NONE of my other females ever experienced this after being spayed. Don't know what's up, but obviously this is NOT a rare thing now