The most common reason for what you describe is due to stretching of the tissue under the breast, similar to pulling a muscle. This could result from having poor support from your bra, especially from large breasts. It can also be the result of trauma to the area r intense exercise without proper support. However, there are other possibilities, including underlying chest pain which could be from the heart, lungs, or esophagus. This is a complicated area and should be evaluated by an internal medicine specialist who can help figure out the cause of the chest pain. Most of the time it is easy to diagnose and treat, but it could be serious, so you should see your doctor. I hope this helps answer your question, please let me know if you have any more questions.