Post pregnancy secondary amenorrhea: what to do?

Good evening, I am writing to you about one of my problems! I am 36 years old and I state that I have always had a regular cycle. I’ve always been thin, 43/44 kg. 17 months ago I gave birth to my baby boy. Immediately after giving birth from 54 kg I started losing a lot of kg. Until today, my current weight is 39 kg! I can not put on even a pound! I’m never hungry, I wake up tired in the morning! Since giving birth 17 months ago I no longer see my menstrual cycle! I have heard from several gynecologists and they all told me that I have a pituitary block! I’ve tried tons of hormone treatments! But without results. How can I go about regaining my weight? How is it possible that after a childbirth there is a secondary amenorrhea? Thank you