How do I even know what's going on when I don't have a period

I was on the depo shot for about a year I got off and three months later I had received my period then I decided yeah it's time for me to get back on the depo cause I don't want an oops lol well I finally made the decision to get off of it completely and now it's been past almost four months and still no period my obgyn says that it's completely normal but how do I even track when my period is when I haven't had one yet dose anyone have any advice?!?!?