Since you took the Plan B within 72 hours of the unprotected sex, then according to Plan B's website, you have an "up to 89%" chance of NOT conceiving.
Also according to their site, it's not uncommon for women to experience, among other symptoms mentioned, breast tenderness and nausea caused by the Plan B.
If I were you, I would take a deep breath and know that you took the steps to prevent pregnancy, and just try to relax in that. Any time any of us have sex, even if we're using the most effective form of birth control, there's ALWAYS a chance of conceiving.
But right now, the odds are very much so in your favor, and there's only a slim chance you'll get pregnant from this.