Can Plan-B Pill Effect Periods?

I am a first time user of the Plan-B pill, well the generic version.
On Saturday morning i finished my period; so , me and my boyfriend had unprotected sex, and he knowingly, ejaculated inside of me on Saturday night; or early Sunday morning at 5:23 a.m. . I thought of the Plan-B pill in the morning, which would be Father's Day (that Sunday morning). I did as the directions said, take 1 pill then 12 hours later take the second one. Well Monday,Tuesday,and Wednesday i did not bleed. Well on Thursday i bled, and so on until this Sunday. What im asking is why did i bleed late, on these days and it was a brownish color, and it was like a light period, but brown and light. I always have heavy periods, therefor i know it wasn't my period. So i was wondering what might this cause be, and is it possible that it's a sign that i might be pregnant?