The chances are slim I would say. I have abnormal cycles and suffer from secondary anovulation. I have a condition called PCOS (Polycystic Ovarian Syndrome). This causes a hormone imbalance. My cycles can be 28 days apart but also have been 6 months apart. With no pregnancy. I would contact your doctor to see if you need to run tests. Melanie if you know your body and you know that it is not normal for you to skip ovulation (if you can truly tell when you ovulate) then in my opinion you should take a pregnancy test.
Sperm can live for up to three days inside the women. And there is no way to tell when you are ovulating unless you take an ovulation test. You can only get pregnant during ovulation. I'd say there is always a chance of getting pregnant, especially if you aren't using protection. The chances of getting pregnant while on your period are slim, but it can happen.