I, m a 21 yr old with a normal cycle. This month I didn't have any pms symptoms and bled heavy (I even had blood clots) for 2 days then stopped. Was this a shortened period or heavy implantation bleeding?

Heavy period? Haven't heard of "heavy implantation bleeding". Possibly an internet phenomenon? If worried about pregnancy, always use reliable contraception properly, and do a home urine pregnancy test 1 week after missing a period or 1 week after starting a "weird" period.