4 Answers

Because you do not know how to deal with the situation or answer the problem- and this creates stress in both your body and brain.  This stress can motivate you to think through the situation or problem or it can make it harder to think.

When we can solve or pacify a situation, we remain calm.  When the questions, situations, problems, and consequences are more difficult to solve, or time constraints require us to respond without thinking things through as thoroughly as we like, frustration increases. With it comes stress, nervousness, irritability. 
I took a series of tests this morning.  One of them got progressively more complicated until I would have had to have a B.S. in Engineering to understand. As I watched my clock (which I hadn't done during the other test), I became more confused with the questions and felt the pressure of not knowing and time running out.  
 

Hormones...........

A mental health professional may know of drugs or dietary conditions that produce the condition you describe and may know how to help you stabilize your thinking when subject to stressful situations. There is a mental health clinic nearby where you live that can help.  

When I am under a lot of stress and in a hurry, my ability to stay on track with what I am trying to do becomes confused and disorganized. I’m sure that makes me look pretty weird if I were in public, As is, this bit of insanity has helped me understand the world better. I sit down and meditate which helps. There are also drugs that help keep me on track and  I find daily vitamin supplements help. The loss of control of one’s own faculty is very disturbing and many,  apparently, have no idea how to properly treat the problem.