Why do I get migraine when im on my period?

Hormone shifts. A menstrual migraine is an unfortunately a common occurance. Many women will get a headache that is triggered by the change in hormone levels. Some women will use non-hormone methods to treat the headaches, others will use a daily hormone and skip the placebo week to reduce frequency of headaches.
Menstrual migraine. Menstrual migraine is quite common and is associated with the drop in estrogen level at the end of the menstrual cycle. These type headaches usually start just before the period begins and stop after a couple days of bleeding, when the estrogen levels begin to rise for the new cycle. This can sometimes be treated with birth control pills or with supplemental estrogen just at the end of the cycle.