Plan B is most effective if it is taken before you ovulate as one of the ways it's thought to work is by delaying ovulation. It may also stop a fertilized egg from implanting but it doesn't always. And it needs to be taken within 72 hours (which you did) but the earlier you take it the more effective it is.
There is a chance that you can still be pregnant.
You probably need to take a pregnancy test if you don't get your period in the next week or so. Might be a bit early to get an accurate result at the moment.
I don't know the limit for time to use the plan b pill before it is ineffective, but if your egg was fertilized before you used it then you can not undo that. The plan b pill is not an abortion pill, and only works if you aren't already pregnant. Wait to see if you miss a period, take a test, and get some condoms/birth control.