What Causes Irregular Periods?

I am 31, 146lbs, 5'8", 3 kids, in good health, and have started working out 5 days a week since the beginning of May 2011. All 3 kids were vaginal, 2 natural - the youngest was born in March 2010 and I had my Paragard placed in Oct 2010. May 24th my period started and it seemed to be early. My period just started again today, June 16th, 3 weeks later. This is abnormal for me as my periods are usually around 28 days. Why would my normally, normal cycle suddenly shorten by a week?