If youve had no history of mental illness before, then it might be from the med.
Ever do a full body cleanse, or a fast?
Dextromethorphan (the DM) can act like an hallucinogenic - but typically in large doses. It sounds like you are very sensitive to the drug. It shouldn't cause any long-term effects - especially in regard to anxiety. But you might have had a bad reaction to the drug, and you might want to talk to your doctor about this. Have you suffered from anxiety in the past? If so, it's possible that having these hallucinations triggered an episode of anxiety. Either way, you should speak to your doctor and let him/her know what you are going through.
All the best..