Explanation for blackout memory at school?

Yesterday during math class I went over what was done from the last class and didn’t remember what we had done, even though the work was right there in my handwriting. My teacher briefly mentioned things as well from the last lesson that I didn’t recall learning at all. I was definitely present for that last class as well. I can’t think of any reason why I would forget that. Maybe the amount of stress? This has never happened to me before and I’m very confused.