Get Pet Questions Answered by Pet Experts
It could be a virus. It could also be a parasite problem, even at this age. I suggest that you see the vet for a fecal exam so that you can find out what parasites you are dealing with an get an appropriate medication.
Until you can see the vet, you can also try changing her diet to a bland one. Feed ONLY boiled chicken (no bones, no skin) and white rice. Most dogs like this and it helps to settle the tummy.
Let me know if you still have questions.