I have a pretty hard time biking (or doing anything strenous) during that time of the month. I always thought it was hormones because I can't remember having these issues when I was on BCP; I never thought about a deficiency. I've had my iron level tested before and was told it's fine (I'm a non-meat eater) but maybe it drops really low during that time. I have supplements but I'm horrible about taking them - this month I'll try to take them regular and see if that changes anything.