What Causes Delay In Periods If Not Pregnant?

Hi Doctor, I have been married for last 2 yrs. My last periods were on March 16th and i dont see any clue of it this month! I tested negative thrice on HPT. No signs of pregnancy. What may be the reason. I am worried. Went to a doctor. She suggested me to wait for another week and then prescribed some scans and medicines. Please advice. I am worried.