I'm usually completely irregular... then 2 years ago they found I had an underactive thyroid which is what was causing it. I've been a lot more regular since, but I still have bouts of AF every 6 weeks or so. And just a couple of months ago, I missed 2 periods... wasn't preggers, but then I had AF, and 4 weeks later had another one.
I'm now due again, but no signs of it - so not sure if I'm pregnant, or if I'm irregular again >.<