I have not taken Plan B One-step, but nausea (13.7%) and lower abdominal pain (13.3%) are common side effects. You should be fine, but if the pain persists and makes you very uncomfortable, it might be a good idea to see a counselor, healthcare professional, Planned Parenthood, etc. How did you get the pill? You could contact your source too.

More information about Plan B One-Step:
Plan B One Step Emergency Contraception