My period late

Winnie

My period is irregular but for the past 2months it been coming every month except for last month. I didnt see it anymore. Me and my boyfriend been try to get pregnant for 9months but I dont know my ovulation date to calculate it.

Also, I been have symptom of tight stomach. I took a pregnancy test and it came out negative. Do any want have advice for help me calculate my ovulation date with irregular period?