What Causes Low Menstrual Bleeding And Brown Discharge After Plan B?

I stopped my depo shot in august, I got on the pill instead when my shot was due. I missed a few pills like four then I had sex after I had sex that time I went and got the plan b. My period came in September and it was normal.this period is not normal at all, I have very little bleeding and brown discharge. After taking the plan b my boyfriend and I had sex alot, could I be pregnant?