Well, first off, is there a reason the dogs haven't been spay or neutered? You keep intact dogs like this and are not vigilant it could certainly result in a pregnancy!!
When the male dog mates with a female, there is a bulb in the penis that swells inside the female and they"tie". They can be "stuck" together, so to speak, anywhere from a few minutes or much longer. Trying to separate them during this time can result in injury, especially to the male.
Do you know where she is in her cycle? That can vary but the most fertile period is usually around 10-15 days. Has she been flagging or receptive?
When you saw them, was he already on her or did you see him mount her?
If she yelped, the Bulbus in the penis may have been swollen, check the male to make sure he is not injured and the penis in back in the sheath.
I would need a little more information but, yes, there would be a slight chance she could have been bred. Depends where she is in her cycle.
Sounds like you certainly don't want a breeding but if you are going to keep intact animals, you need to be more vigilant and keep them separate. Her estrus can last up to 3 weeks, generally.
What breed dogs are these?
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