My boyfriend and I had sex on the 11th and took the plan be 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. 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. Through 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 heavy and then get lighter. I have never had a period this light. Is it just because of the plan be or is it inplantation bleeding?

Chances are that it is the plan B pill. That will screw up your hormones for a month or two. It is a high does hormone pill that is mean to prevent pregnancy, I can't remember the exact mode it does this, but that also means that the percisely timed and tuned cycle that you're used to is going to get screwed up.

It will calm down in the next month or two. During that time you would do well to investigate other forms of birthcontrol. Using the emergency contraceptive pill is for emergencies, rape or other birthcontrol failure for example, not for everyday contraception.