Missed period but negative pregnancy test

So my boyfriend & I had unprotected sex September 25. He cum inside and I took plan b September 27. My period is three days late . This is not my first time taking plan b , I take it and my period comes on time . When I took pregnancy test and I put pee on stick it was blank at first. I had put more pee it came out negative . Idk did I take a false pregnancy test