Postpartum Exclusively Pumping

Susu

So I’m 4 months postpartum. I got my period 3 times already started at 7 weeks postpartum. The last time I got it twice a month every two weeks and now I’m 6 days late. I took a test couple days ago and it was negative. I took the early response digital. This morning I’ve been having orange discharge and backache. I don’t think I can be pregnant though but I just want to make sure. Has anyone experienced this and had irregular periods while breastfeeding or pumping?