Irregular

I'm always the same weight. Sometimes I get af sometimes I don't. I can got 5+ months without it and other times my cycle is only 35 days. I'm underweight and I know you need 17% min body fat to get your period, but even times when I weigh more than I normally do I don't get it until I'm back to my normal weight. My weight fluctuates constantly but I'm confused on why even when I weigh 3-5 pounds more than normal I'm not getting my period. Like I said, I'm UNDERWEIGHT so weighing more I would think id get my period?? Ugh!!