It is also worth mentioning that Plan B is labelled an "emergency" contraceptive for a reason. You're not supposed to take it as casual birth control, it is too hard on your body. Please get on the pill or get an IUD if you and your boyfriend are going to have sex. It is much easier on your body and more sure too, since Plan B does not always work. If you don't know a guy's sexual history or his faithfulness, you should have him wear a condom as well (along with your contraceptive method) because while the pill or an IUD will protect against pregnancy, it does nothing to protect you from STDs. Please be more careful.
Plan b works well to stop us from getting pregnant but the after affect is that our cycle is messed up for 2 to 3 months. That's normal. It's a big dose of hormones (how it works to prevent pregnancy) and they linger. So being late, bleeding in an irregular way are all normal. You can double check with a pregnancy test if you want but what you describe is normal plan b after effect.