Because there was no warm up. If you start a sharp move forward without a preliminary warming-up, the body no time to readjust. Side pain while running is a signal that the capsule of the liver or spleen are filled with blood that no time be evenly distributed throughout the body. It is because of this pain occur.
Because the liver becomes swollen from blood flow. When a person is in a state of uniform motion or rest, some of the circulated blood is not involved in the process. It is in the "reserve" in the liver. The body does not need to handle a large amount of blood, so it is deposited in "reserve". Time running fast, the liver begins to explore these reserves, forcing the blood out of "stock" to circulate. But due to the lack of pre-dispersal body does not have time so quickly to "drive". The liver from the blood flow swells, starts to press on other organs, this is the reason that experiencing pain while running.
Because recently people eaten. After a meal it is not recommended for Jogging. First, the liver begins to participate in the digestive process, and, therefore, the blood flow in it and without that increases. Secondly, running after eating makes your body work hard to digest food, and circulate more blood to working muscles. Because of this, there are often unpleasant stabbing sensation in right side when running.
Because there are diseases of the spleen or liver. Pain in left side when running can be a signal that there are some hidden diseases of the spleen and the right liver. Some diseases do not manifest themselves in the initial stage, but during the violent exertion of the affected organ may indicate that there are some problems. For example, if after a preliminary thorough warm-up while running in my side still hurts, it is advisable to contact the doctor for verification. It is possible that some disease is at an early stage.
Because the person breathes incorrectly. During running, the body is particularly strong needs proper supply of oxygen. For example, experts recommend to breathe evenly and steadily. Such breathing exercises while running, the blood will be oxygenated properly without jerking, and, therefore, all the muscles and organs "will be filled" evenly, and will not cause pain.