Not on periods due, got a brown discharge post a week of taking plan B. Is this common?

My boyfriend and I were getting close but we did not have sexual intercourse I am mainly worried about precum. So the next day I took plan B one step and i had also recently just started on birth control a few days before and about 4 days later after i took plan b I had a period that was a brown discharge , no red. It lasted a little over a week. My actual period was going to start in about 3 weeks. I havn t had many symptoms other than the discharge...maybe a little abdominal pain and gasses in the stomach . Now im expecting my period in about a week and hoping to get it! Is this normal after taking plan B?