What is the pain I'm having under my right breast that's been hurting for about a week now?

Two structures. lie under the RIGHT Breast, RIB and INTERCOSTAL MUSCLE ...Assuming no INJURY, intercostal strain/sprains result from many NORMAL activities ...lifting, carrying..COUGHING Apply Heat and take some Tylenol Hope this helps! Dr Z.
Strain. You likely have a strain of the muscle between the ribs that can take over a month to heal. See your provider of health care if it worsens or persists.