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Hello. Thanks for the question.
The stretching doesn't mean a whole lot. If she's flagging her tail and presenting like she's in heat, then you probably missed her ovulation. If it's just stretching, then it's probably unrelated.
If you want to make sure the mating took, you can get an ultrasound in 2-3 weeks to look for fetal puppies. If the ultrasound is negative, then you'll have to wait until her next heat cycle to breed her. Since she seems like a difficult one, doing progesterone testing will help you time it properly. If she won't stand to be bred, then artificial insemination would be an option for you. There are many vets that take special interest in reproduction that could help out.
I hope this helps, let me know if you have any other questions.