Help me please

So a week ago I had unprotected sex with a man who is just a friend , well he cummed inside me . So the next morning I took plan b , my period was supposed to start Saturday , then today I went to the bathroom this morning there was still nothing , went to the bathroom at work there was a small amount of pinkish blood . Then when I went to the bathroom again it was all brown blood. And not on my pad . My first day of every period I have had is usually very heavy and bright red blood . Could this be a sign I am pregnant ? And the plan b didn’t work ? Please help