Spotting?

So about three days ago I noticed dark discharge in my underwear. I brushed it off as my period starting and threw on a liner. I had the same discharge, color and all, for about two days and it's completely stopped. According to the app, I'm late by 4 days. should this be something I need to worry about? I'm not on any BC or medications that could throw off my cycle, that I know of. ease help!