Sex on 3/17, plan b 16 hours later. period due on 3/21 and got it right on time. does this mean plan b worked?

Yes: Having your period on schedule indicates that you are not pregnant. Plan B is ~95% effective in preventing pregnancy if taken within 24 hours, and ~89% effective if taken within 72 hours. Plan B is should not be used repeatedly. You should start a reliable form of birth control as soon as possible, and should always use condoms to protect yourself from STD.