Stress itself is caused when we have to deal with something we don't feel we are physically or emotionally able to handle and an example of a stressor could be a big test, or moving to a new city.
How this comes into play is when we have to deal with one of these stressors, we go through a stress response commonly known as fight or flight. You act differently depending on the stressor, but you analyize the situation and then your body and mind decide if you can stay and deal with the threat or if you should leave to avoid it. If your body decides you should choose the fight method you get a big burst of energy to fight the potential threat, if flight is chosen you will try to find the easiest and safest mode of escape.
Keep in mind that this is a very primal thing and is triggered by tons of stuff, but most of the time it is a false alarm; an example could be the test anxiety some people feel which is not an actual physical threat, but sometimes yur mind will treat it as one.
The blood vessels dilate to increase the "blood traffic" through the lungs to carry more oxygen to the muscle. The lungs work harder (faster respiratory rate) to increase the rate of oxygen/carbon dioxide exchange in the lungs.
The answer for this question would be all of the above
It would be best to take it to a vet and ask if he has anything on him, or if he is going through some sort of emotional state
