I guess there is always a chance you could be pregnant, Plan B is not 100% effective. But it is pretty effective. The plan B website says that you should expect your period around its normal time, it is also normal for it to be a few days early or late as well. The site also says that if you are more than a week late, or have not had a period within 3 weeks of taking plan b that you could be pregnant.
They are also very clear about the effectiveness in women who are overweight. They warn that plan B is less effective in women that weigh 165+ lbs. and is NOT effective in women weighing 176+ lbs. If your weight is above 165 lbs there is a chance that the medication might not have been able to work for you.
If more time passes without your period, then try another test. If it comes back negatve and you still have not had your period, then you should see your dr about the issue.
It's possible that the plan B delayed your period (it's a high level of hormones). You should probably check in with your doctor to be sure there is no cause for concern.