Following up with bgse;
The big No-No for cats is an artificial sweetener called Xylitol. If that chocolate milk contained any of that, then I would take your cat to the vet because too much Xylitol can poison your cat. It also can poison any dog that ingests it. This is why you should always watch your pet around chocolate, gum, or any water flavoring.
However your cat may have vomited due to the sugar content.
According to Bondvet.com, "If your furry friend eats sugar, you might see vomiting, diarrhea, and discomfort. Symptoms can range from a mild to severe, depending how sensitive your cat is and what they ate. Plus, in the long-term, sugar consumption can cause weight gain, diabetes, tooth problems, and other health issues."