Well, irregular periods do happen and I'd say thus far, you've been really lucky to
usually be regular. I had a period
of time that I had really long cycles due to
my thyroid being slow. I also produced too much estrogen at this time. For me, it self corrected thank goodness. But during that time, my period came every three months! Ugh, the pms was long and drawn out. This went on for about a year or so.