Unprotected sex day of ovulation. Next day light cramping/spotting. Today, day two, severe cramping, heavy bleeding, and nausea. Why?

Wait and monitor. your symptoms, then check a pregnancy test a week later. If negative, wait another week and take another pregnancy test. If you still have cramping/bleeding, and nausea, then go see a GYN to make sure you don't have a miscarriage.