Girl you should now remeber that you are not pregnant, but that the "Plan B" messes up your hormones and that could be the reason you still have not had your period!!
Please use protection as the "PLAN B " is not really good for you!!
Good Luck
Plan B can really mess up your cycle. It's a HUGE dose of hormones. I would go to your gyno or local clinic to get a beta hcg blood test (pregnancy test through bloodwork) and speak to them about using a reliable form of birth control such as an IUD, birth control pill, patch or shot an using condoms as back up to prevent pregnancy.
Don't be alarmed by the lack of menstruations after taking plan b, this is a common side effect. If you're still worried and haven't started your period after another month, you should visit your doctor or local clinic for a blood test, which is more accurate in detecting pregnancy. Also, plan b shouldn't be used frequently or as a source of birth control (>2x/yr), therefore use another form of birth control like condoms and/or the pill. Hope your cycle returns to normal soon.