What Causes Vaginal Spotting With No Regular Period?

I have had an abnormal cycle as far as length between periods go, ever since I got off the pill 2 years ago. I was expecting my period to start on Friday, Saturday or Sunday of this past weekend, instead I have had very light spotting since Friday. Only when I wipe do I notice. I have a pantyliner on and it stays clean most of the time. Is this caused for concern?