The woman gave birth 2 months ago via cesarean section, she's breastfe…

The woman gave birth 2 months ago via cesarean section, she's breastfeeding and about a month ago started feeling her uterus ache like during periods but lightly and not often, I can't figure out what's going on. Has anyone experienced this? She saw a gynecologist in late August and had an ultrasound, they said everything was fine.