Your period can be affected by a number of things. I had a normal period all of my life and started missing them every few months due to a number of things.
-stress from trying to concieve. (couples tend to have a hard time concieving when they try because of unconsious stress you put your body through worry about when to have sex and if that dreadful period will come or if the test will be )
-weight loss is a biggy even just a few pounds!
-getting off the pill
-not enough sleep
-changing work shifts
-hormonal imbalance (when you are very young or getting older)
-starting a new diet
-excesive excersise
these all affect your period in one way or another. Don't worry, if 6 tests said your not pregnant-your not. My periods are usually 31 days apart. Yours may just be changing their length as your body is changing.
Hope this helps....

Sorry, overlooked where you had written "pco". I have no idea what that is. Please specify. Have you been checked for endometriosis? Can be serious. Cause severe pms. I have severe pms, but no endometriosis so don't worry. My heart goes out to you. Hope you find out what the prob is.