Many things can explain a sore breast on just one side, but the important message is that you should speak to a doctor
who can actually examine you. Some of the common causes of breast tenderness include growth associated with pregnancy (although this is usually both breasts), hormonal changes that can vary with your menstrual cycle, localized infections or pressure from a clogged duct (which is especially common for mothers who are breastfeeding or who have recently given birth). If none of these seem to apply to you, then there are other possible causes as well, all of which can be discussed with your primary care doctor
or OB/GYN in detail. He or she may feel the need to get some radiographic imaging, depending on your symptoms, which may either include an ultrasound or a mammogram
. Screening mammograms and breast examinations are an important aspect of female health, and should be performed regularly.
If you are a man, the possible causes will be different than if you are a woman, and trauma or exercise related pain is probably one of the most common. Either way, please speak to your doctor.