What Causes Sudden Change In Postpartum Period From Light To Dark Brown?

Hi, my postpartum bleeding was really light pink/brown a few days ago almost gone. then at 5 weeks 4 days postpartum i suddenly had really dark brown almost black discharge then 5 hours later it changed to a really bright red kinda like a period. What is it? Is this normal or shuld i call my doctor?? I ll be 6 weeks postpartum this friday March 6th