I am not an authority on this situation, I have never been a Teaching Assistant and am certainly not a professor. I am merely a student of Literature much like you (this is my final year!), and this is just my input based on my own school's policies for academic honesty (yours should probably have their own policies that you should check and see if they match up with mine).
It's not a matter of plagiarising yourself as much as it's a matter of "cheating," as defined by my school at least. One of several possible examples of cheating given in our policy document is "Submitting the work one has done for one class or project to a second class, or as a second project, without the prior informed consent of the relevant instructors," which is essentially what's going on here. It's pretty dumb, all things considered. I mean, you wrote it, right?
It would be an absurdly, ABSURDLY harsh punishment over this to nail you with any serious disciplinary penalties (automatic failure of the course, mark on your record, etc etc etc.) However, if your school has a provision for "cheating" by reusing material between papers, it is probably within your professor's power/authority to dock marks from the paper, and you might be at their mercy. Hopefully your prof is cool and it doesn't come to that!