Why do I get migraines after my menses usually. The severarity varies. How can I prevent them or at least decrease severity. Is this caused by low iron, or stress at this time.

It can be. Menstrual migraines may be result of heavy periods which can lead to anemia. Some women don't realize there period is too heavy because they have bled that way for a while. If you change pads.
There . There is a medicine called frovatriptan that is often used in this situation. It lasts a very long time, but has to be taken 1-2 days prior to anticipated menses. Obviously this only helps if your periods are regular. This medicine is not for people with cardiac problems and some other conditions, so you should discuss this option with your doctor first.
No . No one knows why some women suffer from menstrual migraines. Stress & hormonal variation are thought to be possible triggers. Prevention/treatment depends upon whether you're interested in becoming pregnant or not. If you don't want to become pregnant, the answer is easy. Don't have your monthly menses. In other words, take your oral contraception without a break. Instead of taking the last week or so of sugar pills, start your next month's pack immediately. Or switch to one of the continuous versions that purposefully throw out the sugar pills for you. If you do want to become pregnant, or can't take hormonal contraception, i've listed a few links below to some organizations that have very nicely enumerated the traditional non-hormonal preventive treatments.