What is actually happening in the hall

The exam starts. Somewhere in the first minutes an appraisal lands: this matters too much and I am going to fail.

Two things follow. The body responds, which is the heart and the nausea and the hands. And the mind starts commentating, which is the worry, the self monitoring, the running estimate of how badly this is going and what it will mean afterwards.

The commentary is the expensive one. Working memory is a small workspace, and the commentary is using it. So the question in front of you gets whatever is left, which is why the material you genuinely knew on Thursday will not come. Then the performance drops, which confirms the appraisal, which turns the commentary up.