Its not very serious to swallow a few sips of a moldy drink as our stomach acid is strong enough to kill most pathogens. You can expect to have an upset stomach, abdominal cramps, and perhaps diarrhea as well.
There are certain signs and symptoms that warrant medical attention you should watch out for. If you start experiencing nausea/vomiting, unrelenting or worsening abdominal pain/cramps, tenderness of the abdomen to palpation, fever, chills, bloody diarrhea, or inability to eat, you should see your primary care physician or go to your local emergency room as it could be a sign of a serious medical condition.