It could be that she was spayed, but they goofed the surgery. Our Marigold was spayed before we adopted her (has a bad scar from a badly sewed up operation), but she still goes into heat 1-2 times a year. We recently started Feliway, and that has done wonders as far as behavioural issues are concerned.

{{Hugs}} to you -- I know how frustrating a cat in heat can be! and I hope everything turns out well in the long run!