What Causes Light Periods With Brown Vaginal Discharge After Taking Plan-B?

Okay, I had unprotected sex about 2 weeks ago (12 days to be exact) & I took a plan B pill roughly about 6 hours later. I didn t get any side effects from it. But a week later, I got my period, but it only lasted 2 days. & I have had brown discharge when I wipe for the last 2 days & I don t know what this could be. Will you help me find out what this could possible be?