Does Plan B Make Your Period Late?

My boyfriend and I had unprotected sex on the 16th, and I'm supposed to have gotten my next period on the 24th. I wasn't fertile at the time and I took the pill an hour after we had sex. I also took a pregnancy test and it came back negative. I've been taking about 6g of Vitamin C to try and induce my period and it hasn't worked yet, but it's only the second day of taking it. Should I be worried or does Plan B make periods irregular? Mine is usually like clockwork. Do I need to take another pregnancy test in a few days if I don't get my period by then?