Grey tissue discharge immediately before period

Hi ladies! This morning, when I went to use the bathroom, I had a clot of grey tissue discharged—around the size of my finger. It was large enough for me to feel it being “passed”, then as I wiped, I noticed I was bleeding. I’m not due for my period for another 6-7 days so that was surprising. My first thought was maybe I had a miscarriage, but I took a pregnancy test I had laying around, and it came out negative. I also don’t have any weird symptoms (discharge, smell, itching, etc.) so I don’t think it’s an infection. Any thoughts on what this might be? Not sure if I should be worried and/or call a doctor. Thanks!