Flow change

Just curious what would cause flow to change. My periods were really irregular prior to being pregnant, then after pregnancy I breastfed for 18mon without getting it and my son weaned at 19mo I’ve have 3 periods since then ( the last 3 months) the first 2 were terrible heavy super heavy flow like bleeding through ultra tampons and super pads. This last one was barely anything was using lite flow tampons/ liners and they barely had anything..

I’m not complaining (I hate havin a period) but

Why the drastic change? Not sexually active, not on bc of any kind.