See it first happened really early in the morning today
It is not that uncommon for dogs to vomit yellow bile, which is usually a sign of an upset or empty stomach. Dogs will often graze on grass then throw up to help relieve an upset stomach. This can often be remedied by feeding smaller meals more frequently but not more food.
Have you changed foods recently?
This could be something more serious, of course. Any chance your dog may have ingested something in the house or outside? Is the dog eating, drinking, any vomiting or diarrhea, eating and drinking?
If this continues and especially if the dog shows any other signs, a trip to your Vet is warranted.
No one here will be able to answer that question. Did she eat something she wasn't supposed to? Your vet will be able to help you find the cause and give her medicine to stop the vomiting.