Depo Shot side effects

Hi! So, I first tried Depo back in 2012-2013. I had no serious side effects other than not really having an appetite for a few days and no period whatsoever. I got pregnant with my daughter August of 2014. After having her May of 2015 I wanted to get the implant (Nexplanon). I got it removed 2017 to give my body a break. So back in May of 2018 I got back on the Depo shot and I’ve been randomly bleeding and spotting for days and having moderate cramps like a period. The thing is, this has never happened before! With any birth control I been on, I’ve never experienced this. Do anyone know why?