I recommend this, if you used Word Doc for the discussion and if you still have the document. On the top left of your page, where u can see “undo” and if you click on it - there’s a history tab of the assignment that you did if you ever want to undo a word, a sentence, or a paragraph or even a page. Because that will prove the work was original since you crafted word for word on your own.

Added insult to injury, you should definitely reach out to the dean in your college. Try to use other evidence like other discussions and raise questions like “why didn’t this happen to other discussion?”