Blood, sweat, and tears: How exercise affects your period

You've been exercising for weeks.  You feel good, and ready to take on the world (or, you know, the next game/meet.  Same thing).
...You're also set to start bleeding any day now.
If you've just recently started exercising, the truth in the matter is that you're probably going to notice a bigger difference in your cycle than someone who has always been active, simply because for the latter the type of cyclical changes exercise causes are routine, so I'll start there first.
You burn a lot of calories and energy exercising, and your body uses up a lot of nutrients.  In your period, you also lose nutrients.  If you don't compensate and eat more calories during the day, this can cause a condition called amenorrhea, where your period stops.  (Note: If you've just started your period in the past year or two, skipping a month here and there is normal.  I'm talking long term or skipping your period after it's been regular for awhile).  Thankfully, amenorrhea does not cause serious long-term effects if treated quickly and can be easily amended by increasing you caloric intake and decreasing the length or intensity of your exercise.
If you're exercising more, you're probably also drinking more water (or at least, you should be!).  This can help get rid of the acne caused by your hormones going nuts and can also help with cramps.  And, in my experience, the more water I drink, the shorter my period is- about 5-6 days instead of 7.  This isn't scientifically backed up by anything and even if it was I couldn't guarantee it'll work for you, but hey, the less blood the better!
Besides water, exercise itself can also help with cramps.  It increases the bloodflow to your uterus.  So grab that water bottle, lace up your shoes, and go take over the world your favorite sport!