bleeding PP

i’m 5 weeks pp and breastfeeding, at 3 weeks i started heavy bleeding and cramping after lifting some heavy stuff around and it lasted for four days so i ended up going to the emergency room, they told me it was most likely just my period, now at 5 weeks pp im bleeding and cramping again after doing a lot of cleaning and lifting heavy stuff, is it really just my period happening again or is something else going on? i had an ultrasound done and it showed a couple cysts but nothing major but i have such bad cramps all the time and my lower back always hurts