Is this how periods work after "Plan B"?

Me and my fiance had sex Sunday, then the condom broke, I took a "Plan B" later on that night since I knew I was going to be ovulating soon. Now currently, I'm not on birth control because of some health issues.

Friday, I ended up feeling cramps but wasn't really paying attention because my whole body ached from a cold that I have. When I went to the bathroom, I saw brown/dark blood on my underwear. I'm guessing it's my period and unlikely me spotting.

I've taken the Plan B a few times in my life. I know sometimes your period can come a week early or a week later but I just got off my period about a week ago. Isn't it too soon?

It just doesn't make sense to me. Also it doesn't look like any period that I have seen.