My kitten vomited yellow fluid for 3 days, spirits worsening, no diarrhea; I fed a little cat treat two days before the vomiting started

Based on your description, the kitten vomits yellow fluid but has no diarrhea, which suggests gastritis or a gastric ulcer, and the cat often has poor appetite with these. Take it to the veterinary hospital for a stomach check promptly. In addition, a foreign object in the gut or stomach that the cat is trying to vomit up can also cause yellow vomiting, which also needs an X-ray. To avoid worsening, take the kitten to the hospital as soon as possible.