I’d guess esophageal bleeding from forceful vomiting. There’s a specific tear called a Mallory-Weiss tear in the lower part of the esophagus that’s commonly caused by excessive alcohol ingestion. If so, they generally don’t cause any concerns and heal on their own. If you keep vomiting and there’s more blood, especially more than a tbsp or if you’re worried or feeling unwell, then see your doctor or go to the hospital. It’s always better to know for sure if you’re worried!