
This happened to 2 of mine

so you aren't the only one. We rang our vets the next morning and took her straight down for the Alazine injection, with a second jab at the recommended interval. You must make sure that they are then strictly kept apart as the vet can't give her another injection if they mate again.
Please, please don't let her have a litter at this age, she's still a baby herself and with neither of them old enough for their health tests, the injections are the best way forward.