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You are through to Dr Coles, I am a UK based vet and I am going to respond to your question just shortly.
Thank you for your question regarding breeding. Slip matings can be normal for some dogs. Sometimes this can be due to the female being nervous/inexperienced or not staying still. Some slip matings can even be successful in producing puppies. I would suspect this is due to the bitch being inexperienced. I would not recommend doing an internal examination on a dog if you are not a veterinarian/veterinary trained as you can easily introduce infection into the bitches vagina/cervix. The normal bitches vaginal canal is not much longer than the length of a finger. You may be feeling the vaginal fornix which is a blind ending pouch. I would recommend that you have the female examined by a vet for a reproductive exam and you may need to try again at the bitches next oestrus.