I have just visited a friend with two Jack Russells. My one year old Westie 'knows' them and they get on. This time, the younger one kept trying to mount her. I grabbed her and we kept them apart. It would look, from the dog's behaviour, as though she is in season but I can see no signs. No blood, no engorged vulva, etc.
I'll be taking her to vet tomorrow to get his opinion. She can't get pregnant from a 2 second encounter, can she?
There is absolutely no way she will or can be a mother. I was waiting until she was two (couple of months) to get her spayed.
(From my very limited experience of breeders, it seems to be quite a job to get them to mate. Whereas the Jack Russell (aged 5) seemed to know exactly what he was doing).