Can a 3 day light period w/ light cramps first day be implantation bleeding & my periods are normally 7 days long?

A person can't tell: A positive pregnancy test can tell a woman that PRIOR spotting was actually implantation spotting. Until then, it can be hard to tell if a woman is pregnant because some symptoms seen in early pregnancy can also happen in non-pregnant women. A urine pregnancy test gives a reliable result about 3 weeks after sex (3 weeks after when conception might have occurred), or 1 week after missing a period.