What Should I Do About Missing My Periods For Over Four Months?

Doctor i didn t get my periods for past 4 months last year i had same problem so i consulted a doctor she gave a tablet and massaged in my stomach after reaching home without eating tablet i got my periods then for 3 to 4 months i had a regular periods but now i had same problem what to do doctor iam so sad iam virgin also....iam 18 years old weight 135 height 173 it may due to my weight or something else please tell doctor