I Had Sex Nov.30, And I Took The Plan B

I had sex Nov.30, and I took the plan b pill December 2. My expected period is supposed to start Dec.23 however I started this brownish spotting Dec.18 and it has lasted me 2 days. It started off brown but it got a little heavier in blood but it was a little unusual for me. But it started in the timeframe of my expected period. Is this quite normal? I'm wondering if the pill worked. Was this spotting my period after taking plan b?