Talk to Gynecologist on Under Weight And Irregular Periods

I am 23 years old and I'm underweight since 5 plus years. My weight is 40Kgs and it doesn't go up no matter what or how much I eat. My menstrual cycle is irregular too. I had my Pelvis Ultrasound done previously and its report read normal الحمدللÛ. My last period came on 16th of April and no period uptil now. Doctor asked me for hormonal tests on 2nd day of periods. But there's no periods yet. Another doctor prescribed Duphaston for induced periods so that I may have my hormonal tests conducted.. Should I use this? Or wait further? Any guides where should I focus on my health? Diet? Physical activity?