Is Spotting And Brown Discharge Sign Of Implantation Or Usage Of Plan B?

My boyfriend and I had sex on the 11th and took the plan b pill about an hour after. On the 21st which was a Tuesday i started to have off and on cramps. Then on Thursday they started to happen for longer periods of time. Friday it continued and that night i wiped and had a spot of light pink blood and i thought nothing of it because my period was due that day. The next day(saturday) i had cramps all day long as if they never went away and were more severe than usual , i also felt like i needed to throw up but never did. Throught the day when i wiped there would be a little bit of brown blood. That night i went to the restroom and i had some red and brown blood on my underwear, i wiped and it was the same colors so i put a tampon in thinking i had started my period. Since then the blood flow has been really light but still brown and red. My periods usually start out heavy and then get lighter. I have never had a period this light. Is it just because of the plan b or is it inplantation bleeding?