Hi I Had A Normal Cycle February 23-27. Ovulation Was

Hi I had a normal cycle February 23-27. Ovulation was around the 5th or 6th. I've been having unprotected sex with boyfriend. On the 13 th of march I experience spotting light pink. Which only lasted an hour. Took a pregnancy test negative. Period was due March 22 and did not come. What could this be???