I need answers

So when it comes to my period it’s regular and the app predicts it perfectly. But last month almost 2 weeks after I got my period I started spotting which soon lead to my “period”coming again and being heavy like really really heavy which has never happened before. The period lasted for a good week and a half which is not normal at all,and it went back to light spotting and then stopped.

First question is does anyone know why did this happen? Has this happened to anyone before? I’m not on birth control nor have had sex in a while so what could it be?

Moving on to today my period is supposed to come but it’s 3 days late should I be worried or should I not be due to the fact I had an irregular cycle last month ?