Hey There, I'm Not Due For My Period But I've

Hey there, I'm not due for my period but I've been serverly cramping today and passing large clots about the size of a 50c piece or bigger, my partner and I have unprotected sex sometimes, I took a pregnancy test a week again, negative, but I'm a bit confused cos I'm in agony no bleeding but big clotting, I really need to know what's going on?