It's too hard to sway when you're under weight. My bmi wasn't quite as low as yours but I got down to 16.5 and I had a 31 day cycle (I have had 25 day cycles all my life) my body geared up to O and didn't then tried again 4 days later and I finally O'd but it freaked me out that I knew my body was having trouble ovulating so I dropped the calorie counting aspect of the diet but still skipped meats and breakfast and tried not to eat a ton of sodium. I gained a few lbs and the next cycle I O'd normally and got my BFP! It's just too hard on our bodies when we are underweight.