What Causes Delayed Period After Taking Plan B Followed By Unprotected Sex?

I had unprotected sex Jan 23rd...took Plan B after few hours.Had light brownish disharge on Jan 29th that lasted for 3 days.But my last regular cycle was on Jan 9th.I did took a pregnancy test today (March 2nd) since I still don t have my period which supposed to be here yesterday and the result came out Negative but I still I am worried that I am pregnant.Should I see a doctor?