What Causes Irregular Periods And Brown Spotting During Periods?

hello last month i had my period on 02-11-2011 and i wasn't supposed to start until 02-17-2011. i have very regular cycles alternating 28/29 days and my period is normally 3-6days heavy . My period on 02-11-2011 only lasted a day and was considerably light. My next period was supposed to be on 3-11-2011. Today 01-13-2011n I am having brown; really light flow. normally when i am active my flow increases while i work out. i worked out for a hour and a half and nothing is different. i took a test this morning and it came out negative.