Does Plan B Cause Spotting?

Recently, I took the Plan B emergency contraceptive pill. I had my period a week before taking it the week after taking it i had my period again, thinking its normal.The week after my second period, my boyfriend and I had sex but he didn't ejaculate in me;Today I'm experiencing very light brown spots.My question is the following: I'm I experiencing this spotting because of plan B pill?or I'm i pregnant?