Doesn't hurt to prepare for everything. If you do get accused of plagiarism, and you did not do it, you should keep your cool and realize that you do have recourse (can go above the professor's head). Keep calm and try to talk it out. If the professor doesn't see the light (for whatever reason) you should go to the department chair and/or the dean of the college as soon as possible to get it sorted out.
This is all assuming that you did not plagiarize. There are occasions where people accidentally plagiarize.
In reality, it will probably be something good and you shouldn't worry. But be prepared and don't say or do something you will regret later, if you are accused.
