It is a category
X drug in pregnancy
, meaning it can cause permanent damage to a developing embryo. I don't know how long it can stay in your system so that is probably why you cannot take it in early pregnancy.
Next, call the pharmacy and speak with a pharmacist. Let a professional explain to you why you should not be taking this drug while pregnant.
Third, tomorrow you should call your doctor and get some answers. If they are unsatisfactory you need to find another doctor.