I'm tryn to track my ovulation without buying those expensive ovulation test.. It's hard get the cheap ones because they sell out quick here in my town. So I been doing the cm check. My period last 3-5 days on a average 28 day cycle.. It's like my ovulation skips every month. First it was.. Like the 9-13th, and then next month it was little later, what other methods can I use???