I used ovulation tests to determine ovulation and had intercourse. it's now 5 days later and i'm experiencing heavy bleeding. what could this be?

Uncertain. Possibly: ...it's implantation bleeding, altho this tends to be very light. Your normal cyclic hormones might be a bit confused, causing an early menstrual period. In any case, if you had a (+) ovulation test and are now experiencing unusually heavy and/or painful bleeding & cramping, I recommend you see your doctor. A pregnancy test this early might not be helpful either way. Best wishes!