I've been spotting bright red blood for five days now. i took the plan b pill on the 16th. could this be implantation bleeding or just a light period?

A person can't tell: Wait and test. A positive pregnancy test can tell a woman that prior spotting was actually implantation spotting. It can be hard to tell if a woman is pregnant because some of the 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).