What Causes A Large Fleshy And Lump Discharge During Menstruation?

Hi, I just my second day of my period, which is usually the strongest. However, today, not only was it strong, it had a large lump of flesh that came out with it about 6-8cm in height and 2 cm in width. It can't be a miscarriage as, maybe 2 days before my period started, I took a pregnancy test and it turned out negative. Should I be worried about that lump of flesh? Also, I don't normally get terrible dysmenorrhea, but when I don't exercise for a long time, I've noticed that the pain increases. Is this observation relevant?