Missed period after plan b

So I had sex 1 day after my period finished then right away took a morning after pill. One week later I get another period which is expected when taking plan b, however I am now 11 days late from my regular period. I have taken a digital pregnancy test and came out as not pregnant, I took a normal one today and it also came out negative I forgot about it and left it I went back a few hours later and saw it had a faint line. Could this be a false reading?