Period Postpartum

So I'm 8 weeks postpartum and for the past 3 days I have felt really crappy. Constipated (i eventually pop but it takes serveral attempts throughout the day) headache, nauseated, tired and now for the past 2 days I've been having period like cramps. But so far, no period. Is this normal for postpartum or does that sound more like pregnancy? Took a test yesterday and it said negative...