The most common causes of pain in one breast are a pulled muscle, cyst, or plugged duct due to breastfeeding. A tumor, either benign or cancerous, is also a common cause of pain in one breast rather than both. How likely these potential causes are often depends on the woman’s age, if she is breastfeeding, or if she has been exercising heavily or otherwise participating in a strenuous activity that might pull a muscle. All of these causes have some sort of treatment, if not an outright cure. In many cases, the pain might not go away without treatment prescribed by a health professional, so seeing a doctor as soon as possible is usually highly recommended.
A pulled muscle near the chest, caused by playing sports, lifting heavy objects, or similar activities, can make the breast itself feel as if it is hurting. The pain is normally most severe during deliberate movements, but it can also be painful when breathing. It may make finding a comfortable sleeping position difficult. Treatment primarily involves time because there is no quick cure for a pulled muscle. An over-the-counter pain medication can sometimes provide relief, along with applying moist heat.
Sometimes a fluid-filled cyst develops and causes pain in one breast. Fluid-filled cysts are not believed to be an indicator of cancer risk, as contrasted with a solid cyst. Breast cysts are fairly common and often do not require any sort of treatment as long as they are fluid-filled and not solid. Fluid-filled cysts can be drained if they are causing enough discomfort to warrant it, but are typically not drained otherwise. The detection of a breast cyst often requires a mammogram or other technology-assisted tests, as they are often not detected by hand examinations unless they are particularly large.
Sometimes a woman who is breastfeeding will develop a plugged duct. This happens when milk is not properly draining and can lead to swelling and pain. A plugged duct can lead to symptoms reminiscent of the flu, as well as unique symptoms such as pus and bleeding from the nipple, if it becomes infected.