Suggest Treatment For A Delayed Period With Negative Pregnancy Test

I have been taking birth control for the past 3 years, I have not missed any pills and have never had this problem, but I am now 4 days late on my period.

Im sorry if this sounds nasty, but nothing red has come out, only black stuff. Now I do occasionally have the old blood during the beginning of my periods but it has never been black!

I took a pregnancy test yesterday and it said negative, but I did buy it from the Dollar Tree so it may be too cheap to show up if I was pregnant.