Breasts

This has happened only one before, but when I run with a loose sports bra my breasts really start to hurt around the nipple area. This only goes on for about a bra but during this time it is extremely uncomforable to not be wearing a bra. ( like when I sleep or shower) it is even somewhat painful. I resorted to wearing 3 sports bras for the rest of the day just because it made it more comfortable. What is going I ?

cnn