Hi there
Drooling like this is usually caused by something in her mouth:
1) Stomatitis - Infection of the oral cavity
2) Tonsillitis - often causes coughing as well
3) Teeth decay - although this is unlikely as she is so young
4) Tumours - also unlikely as she is young, but not impossible
5) Bee sting in her mouth - if she is outdoors and chasing insects
6) Systemic infections causing drooling as a side effect (Rabies- depends on which country you live in)
7) Ulcers - also caused by systemic infections (uremia)
Are there any other symptoms? Is she still as active and eating well? It may be worth trying to open her mouth and have a look yourself. If you are not able to and she keeps drooling you should take her to the vet for a checkover.
Hope she will be OK!
Torunn Beurling DrMedVet