Joy is right. I was a week late with my last period and it was due to stress for me.
personally I recommend Clear Blue Digital because I got a positive response with blood hormone levels of only 27
Being late, when not pregnant, usually means something is going on hormonally. This could just be an "off" cycle where all you get is some spotting. If your next cycle starts with the same spotting and no real flow you can call your OBGYN and see what they say.
And you can always retest in a few days. I recommend First Response. Sometimes just relaxing (take a bubble bath, get your mind off of problems by journaling, etc.) can reset everything.