Must I ask my professor if my misunderstanding of his words, which led me to accuse him of something he did not say, is the reason for him giving me a re-examination, and are my new grades considered a sin and invalid?

Lying is religiously forbidden, and it is intentionally reporting something contrary to reality. It is considered a major sin that necessitates repentance. A Muslim is not considered a liar if they convey speech by its meaning or recount what they believed was the speaker's intention. However, intentionally altering speech while knowing its connotation is the forbidden lie. If an alteration occurs in an exam, there is no sin upon the Muslim for that, unless they intentionally committed the lie that led to the alteration.