There can be various reasons for missing your period in absence of pregnancy
and these cause may range from stress
imbalance,rigorous exercises,abnormal weight gain etc.
Your doctor is right in prescribing the medicine to you as this will result in your periods taking place.
He is also right in advising these investigations so that a cause can be found and rectified.
You should always tell every thing to the doctor and nothing should be hidden from him,although it may not make any difference in your current treatment.