A thick, rectangular shaped solid came out of my vagina during menstruation diagnosis what is it? i'm scared.

Clot?: Sometimes when blood clots the plasma portion of blood can look like tissue... And then this can get embedded in a clot and look super scary. I'm guessing that's what happened. Sometimes lining from the uterus can be funny looking too. Rarely there could be a polyp that falls off (unlikely at your age). If you're sexually active, could also be a gestational sac from miscarriage.