It's when the last half of your cycle, the post-ovulatory or luteal phase, isn't long enough to support pregnancy. It typically needs to be at least ten days. (I, however, conceived on a 7 day luteal phase the month after I asked my Bible study group to pray for us.) You should get a basal body thermometer and chart your temperature. Then you'll know exactly what your cycle is doing, when you ovulate, how long your luteal phase is. My cycle was quite irregular, like 25-40 days, but with charting, I always knew when to expect my period because my LP was always seven days.