When you are retested, disclose any medications you took before the test, the technician should be able to determine if they would have interfered with your results.
Depending on exactly which testing method was used, it is possible for drug tests to throw up false positives in a not-insignificant number of cases. You might have gotten caught by a false positive.
A more in-depth test should be able to confirm your story. Good luck, and congrats on two years of sobriety.