Hi XXXXXXXXX~
If there was a successful mounting, then there is absolutely the possibility that your dog is pregnant. A dog doesn't need to get locked to a bitch for a pregnancy to result.
The only way to know for sure at this point is to have your regular vet take a peek at your girl...she's too early in the pregnancy to take an x-ray for confirmation, but they could do an ultrasound.
You could also try buying a human home-pregnancy test and slip it into her urine stream during her first potty break first thing in the morning..but these tests aren't always 100 percent accurate.
I hope this helps.
Certified Veterinary Technician
Associates Degree in Veterinary Technology. State and Nationally certified.