A critical issue concerning stress among students is its effect on learning. The Yerkes-Dodson law (1908) postulates that individuals under low and high stress learn the least and that those under moderate stress learn the most. A field study and laboratory tests support the notion that excessive stress is harmful to students' performance.
Mechanisms that explain why students perform badly under stress include "hypervigilance" (excessive alertness to a stressful situation resulting in panic--for example, overstudying for an exam) and "premature closure" (quickly choosing a solution to end a stressful situation--for example, rushing through an exam).
Subject matter too difficult
It is possible that you took a course where the subject matter was too difficult for you to understand. Sometimes the pace is too fast for you to keep up. Another problem is that you did not learn required material in previous classes.
In college, you are able to drop out of the class if it is too difficult. But if you can't drop a class, seek help. Let your teacher know you are having problems, and perhaps you can get some help with the class. That is why it is good to be selective in picking courses to take.