Well that kind of feeling is not entirely abnormal nor entirely your fault. Studies have shown that examination held at large areas causes intimidation to students, lowers their confidence and inhibits their brains' ability to perform well.
What does mind block have to do with large exam hall, competition and the number of competitors? In these three instances, the brain is easily overwhelmed and intimidated. Due to the many mixed feelings and thoughts, the brain cannot figure out which task to manage first, and so it shuts down and move on. Likewise, when it knows that it won't win the competition - they are just too many competitors - the mind is less willing to put in the cognitive effort.
As depressing as it may seem, there is one way to combat the effects of excessive choices, that is to group items into categories. It turns out that even useless and arbitrary categories make people happier with their choices.
Back to the context of exam hall, a better suggestion to erase the effect of excessive competition is to have people take tests in small rooms, instead of great big lecture halls. The subdivisions can be arbitrary and meaningless, but they might make people less frightened by the prospect of all their competitors. As a final note, when the brain gets intimidated, it typically does not try harder. It just shuts down.
In view of such adverse effects of exam hall which defeat the true purpose of fair assessment, schools, colleges and universities should consider having more small examination rooms. Not only exam invigilation can be done easily, but this also increases the overall thinking performance and possibly increases the number of students getting good grades - the way it should be.