I had sex more than a month ago, took the Plan B pill. I started my period but it suddenly stopped two days later(heavy flow). I took a test and it's negative. What might this mean?

Plan B can cause bleeding to happen--it may or may not be your period.  You may also miss your period as a result of taking Plan B.  It was probably a little too early to take a pregnancy test and have it be accurate.  

Typically, these tests should be taken around when you have a missed period.