Same thing happened to me when I had to take plan B.
Except it also me in a rage for a week as well. I hate that pill. Hope we do not have to resort to it ever again! Btw I do not think that is a rare side effect. It's not like we can just pop a pill with a bunch of hormones and expect it to not have an effect on us.
Has your doctor started you on Iron or vitamin B (depending on your type of anemia)? If so, then you're at least not going to have to worry about that getting too much worse. If it continues for another week or so without getting any lighter, make an appointment with your gynecologist. Unfortunately, Plan B is a huge dose of hormones and our bodies don't always react well to that.