Which branch of psychology deals with decision making under stress?
Decision-making or decision theory is its own subdiscipline under cognitive science (also often studied by statisticians, philosophers, economists, and faculty in business schools). Within this discipline, understanding how stress affects your behavior is very important and not understudied area. For a recent survey with a neurobiological focus, see:

Starcke, K., &amp; Brand, M. (2012). Decision making under stress: A selective review. Neuroscience &amp; Biobehavioral Reviews, 36(4), 1228-1248.

Of course, stress and decision making is not restricted to humans. It is also studied in a purely biological setting. For a survey focusing on predation, see:

Lima, S. L. (1998). Stress and decision making under the risk of predation: recent developments from behavioral, reproductive, and ecological perspectives. Advances in the Study of Behavior, 27, 215-290.