Oxford English Dictionary:
emotion
• noun 1 a strong feeling, such as joy or anger. 2 instinctive feeling as distinguished from reasoning or knowledge.
Is stress a strong feeling, along the lines of joy or anger? Yes.
Is stress a instinctive feeling as distinguished from reasoning or knowledge? In the case of a Ball Python, stress is a instinctive feeling as distinguished from reason. Such reasons may be due inadequate husbandry or housing. So yes.
Therefore: Stress is an emotion according to the definition of the Oxford English Dictionary.