Irregular bleeding

This cycle my period lasted 3 days (which is about normal for me, mine unusually last 3-4 days due to my infertility issues). Period lasted 3 days normal/heavy flow. Then the bleeding completely stopped for 4 days. Out of nowhere today, the bleeding started again, only this time it's slightly different in color and more mucosy. This has happened a few other cycles this year, but it hasn't happened for months. Any thoughts on the cause of the irregular bleeding??