What Causes Brown Vaginal Discharge After Taking Plan-B Followed By Unprotected Sex?

I ended my period on the 2nd of this month and started back on the 16th of this month and ended again on the 20th. The night of the 20th I had unprotected sex and took a Plan B pill on the 21st. Today, the 26th I went to the bathroom and saw brown, maybe discharge? And put a tampon in, I took out the tampon and saw brown with a spec of red blood. What caused this? Should I be worried about pregnancy? My next period should be the 10th of April. Thanks in advance! I m freaking out.