if you have chest pain, generally you should go to the ER right away. However, there are some causes of chest pain which, believe it or not, are relatively benign.
For example, pain from stomach ulcers or reflux of acid from the stomach into the esophagus can give you chest pain. Classically, the pain is burning and related to food. In the case of acid going up your esophagus, the pain is worse lying down.
Other organs in the abdominal cavity such as the liver and spleen are located up higher in the abdomen. Pain from these organs from inflammation (ie hepatitis) enlargement (enlarged spleen from Mono or cancer) infection or injury often will lead to chest pain. Some of these conditions for example bleeding or cancer are certainly not so benign, but in many cases the cause might be something not serious such as Mono.
Pleurisy refers to inflammation of the linings of the lungs (pleura). When you take a deep breath, the lung expands and so too does the pleura. Often this is from viruses.
A bruised chest from even a minor fall will cause pain, as will overexerted chest muscles from say coughing.
Shingles which affects the nerves might lead to severe chest pain, as the shingles virus irritates the nerves near the rash and if on the chest then those nerves around the chest will light up.
The problem is that there are enough serious causes of chest pain so you should in almost all cases just give in and go the ER.