Angina or Angina Pectoris is medical term for the chest pain or the discomfort due to coronary artery disease. Unless you are under treatment for angina and have medications to take, all chest pain should be treated as a medical emergency. Tell someone you have chest pain and immediately seek medical attention.

How does it happen?

When the work load of the heart increases (such as when you start to experience strong emotions, or exercise), the blood flow of the heart increases. Increased blood flow increases the amount of oxygen and nutrients received by the heart muscle, elements the heart muscle needs to work at this new, more demanding pace.

If the supply of blood and oxygen cannot meet the demands of the heart, the heart enters a state called ischemia. Ischemia means that there is insufficient blood supply to meet the heart's current needs. Chest pain or discomfort (angina) is the heart muscle's way of signaling to the rest of the body that it is in the state of ischemia. Angina has been described as a pressure or intense chest pain. Angina usually causes people to stop what they're doing and rest, which promptly reduces the work load of the heart and takes it out of the ischemic state.

You can find further information here: The American Heart Association Website