To be sure, this is not a prophetic dream. Dreaming that you are sick, according to what you said, should be related to the exam. I think this is an escape dream. Because you have failed before, you will naturally worry that you will fail in the following subjects. Of course, there should be the possibility of failure. Out of this anxiety, your subconscious chooses to escape, dreaming of being sick, asking for leave when you are sick, or feeling less guilty because your physical condition has failed. Therefore, don't worry that this dream will bring you bad luck or omen. And suggest doing what you should do. Good luck ~
If in doubt, please ask.