Hi there.
The most common cause for breast pain is breast tissue response to hormonal changes in the body. Sometimes, you may notice a distinct pattern or cycling of pain which intensifies usually just before you had menses, and then gradually declines or resolves a few days into your menstrual cycle.
Musculoskeletal strain would also present with breast pain, especially if you use the arm more frequently than the other. Anxiety would also heighten one's sensation of pain that could add up to the problem.
A mammogram may be a reasonable idea just to see if there are any problems, but I won't be surprised if this turned out to be normal.
Regards.
Dear agapi117: Without evaluation, it is impossible to speculate on what might be causing your discomfort. The most common cause of breast pain is related to the menstrual cycle but there are other things, including gas, that can feel like breast pain. Most breast cancer is not painful. You might benefit from seeing a medical breast specialist. These are usually affiliated with large academic medical centers.