Good evening - I'm Dr. Sara. Dr. Altman has already given some great suggestions to help Kolby get back on track.
When I see young dogs that are suddenly vomiting but acting relatively normally otherwise, the first couple of things I consider are infectious diseases (think stomach flu), dietary issues (I ate something that didn't agree with me) or foreign body obstructions (I ate something that's blocking my GI tract). These can range from mild things that they get over on their own, up to surgical emergencies.
As long as their breathing is not labored, there is no abdominal distention, and their gums are pink and moist, I usually recommend holding off on giving any food or water for at least 6-8 hours. If they are continuing to vomit though this time, it's time to see the vet. If they stop vomiting, then I start by offering a small portion of water. If they keep that down for more than 2 hours, I offer another small amount of water and a small taste of food (1-2tsp). As long as they continue to tolerate small meals, you can gradually increase the amount of food at each sitting and increase the time in between meals. If they aren't eating at all for >24 hours, this also merits a trip to the vet. The pepcid Dr. Altman mentioned can also help.
To answer your most specific question, "What could it be?" the answer is likely "we will never know". Just like us, dogs can get GI upsets that they get over quite easily on their own and we rarely find out exactly what caused them.
Please keep us updated on how she's doing!!