Why can't I retain information?

I have a good memory; or at least I think so. I am a straight A student. Or at least I used to be. This time maybe I’ll have to be happy with Cs. I have to sit my O levels in June and we have our first mock examination on Monday.
I have been studying hard for it throughout the semester. I thought it would be easy and went into my coursework with high hopes since I had reviewed a majority of it beforehand.
I have been studying all day yesterday and today. I have to study all day tomorrow. The exam as I said is on Monday. I know absolutely nothing. I simply cannot retain any information. This has happened to me before. But not at this level. I simply cannot remember anything.
Everything I learned yesterday has disappeared from my mind. Same for anything I have studied today. And I study well. I make recordings of myself essentially teaching myself beforehand. I memorise from summarised notes, and then read the textbook multiple times too. I take small breaks in between topics to refresh my mind. I keep hydrated. I use memory tactics and make acronyms.
NOTHING is working today. Could this just be due to stress? Shall I just keep plugging through my coursework and hope for the best? Or is there something I can do?