What Causes Irregular Bright Red Discharge Along With Cramps?

My period is consistently regular at 28 days. This month, I was 14 days late with multiple pregnancy symptoms, but still got negative pregnancy test results. At 14 days late, I started my period. However, it's abnormally heavy, MASSIVE cramps, bright red, and so far a golf ball sized clot passed. Could this be a sign of a miscarriage?