Irregular periods (Help)

I've never tracked my periods until recently because I'm trying to conceive but before then I was on Nexplanon for 3 years and then I got on Mirena IUD for 6 months or more while I was on birth control my periods never came each month I would skip for couple months before I would get it I looked on google for help I still can't figure out how to calculate my periods it shows on the app I was suppose to start on the 15th and it has came. Can somebody help?

[Sorry if this is posted in the wrong group]