Miscarriage at 6 weeks and now brown blood looks like coffe grounds

So I had a miscarriage the Saturday before thanksgiving ended up being a threatened miscarriage went to the er and she told me no alcohol or sexual activity until I went in and got retested for my hcg levels. I went back and my hcg levels were zero she told me everything should pass on it’s own if it hasn’t already and I was cleared of not doing anything so to say dangerous? (Drinking and sexual activity) so how fast forward to almost the end of December and I am having coffee ground like fluid and I’m starting to get worried that not everything was removed from my uterus? But if I tested zero for hcg then everything would have had to of been removed correct? I haven’t gotten my period yet and Idk if I even ovulated yet since I haven’t been tracking that yet. Any one have opinions on what it could be? Or been through something similar before ?

Much appreciated for responses. Thank you