I drank a small amount of peppermint extract, is that bad?

No though may cause: Peppermint is used as a medicinal herb. It is a natural antimicrobial and is used to reduce cramps and spasms in irritable bowel syndrome. In some people too much peppermint may cause temporary gastroesophogeal reflux (leading to a case of heartburn), so that is the worst that might happen.

Peppermint Oil: Ingestion of too much peppermint oil can cause nausea/ vomiting, abdominal pain. However, it for a variety of digestive issues (flatus, nausea, vomiting, cramps, loose bowel movements, heartburn, indigestion & irritable bowel syndrome).