You may experience another period-- it depends on how things affect you.
Your period
will be a little off. So don't panic if you're late.
The side effects like cramping are very common and can be similar to pregnancy symptoms, so don't panic about symptoms either. If you're late- test. But you took Plan B within 72 hours so you have a good chance of it working.
Now that you're sexually active, schedule an appointment with a gyno and have a pelvic exam, std testing (did you use a condom?) and discuss birth control options and what would be best for you.