Jenn is right. It shows that they are enjoying the petting, and probably want you to pet or scratch the whole way to the base of her tail. I usually draw my hand through their tails also. Male cats do the same thing. It's probably relaxing, like getting your own back rubbed or scratched!
A female in heat does like a lot of attention because she is so nervous, but the behavior would be much more extreme. That is because of the instinct to reproduce, not because of desire. I think females just put up with the act, even though pregnancy is the only way they get relief from those symptoms.
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A dog, I have always said, is prose; a cat is a poem. ~Jean Burden