I am a 48 year old ultra distance runner. On two occasions now, both following 50k races, I have experienced breast swelling. The first time, a year ago, it was both breasts. This time it's just one. The swelling is so profound I can't wear my regular bras - at least a cup size increase. There is no pain, just the swelling and it extends from the bottom of my collar bone to the entire breast. Last year it went away after about a week. Although I'm small breasted (barely a B) I wear supportive bras to run in and I have not had any kind of breast enhancement surgery. My last mammogram was in August '13 and was clear. What's going on here?