If your periods are irregular your ovulation is proably irregular as well.
So don't go by your cycle. Go by the other signs your body starts to give you. When I first got pregnant, I thought I had the flu, I kept waiting for the nausea, the fatigue, etc to get worse or to get better but it just dragged on for a couple of weeks before I even began to suspect I might be pregnant.
The nausea is a normal reaction for your body as well as tender breasts, a crampy feeling in your abdomen. If you have these signs, it might be tme to go to the doctor.