Oops, here you go...
"Costochondritis is a condition that causes chest pain due to inflammation of the cartilage and bones in the chest wall. Also called Tietze's Syndrome, costochondritis occurs when there is inflammation at the junction of the rib bone and breastbone (sternum). At this junction, there is cartilage joining these bones. This cartilage can become irritated and inflamed. Depending on the extent of the inflammation, costochondritis can be quite painful."
Shell, this could be something called Costochondritis. Here's some definition of this condition, but you can go ahead and google for more details. I've had this a long time and it is constant and can be very painful. I feel for you.
If the doctors cant find something wrong, maybe your wearing the wrong size or type of bra. or if you dont wear bras often maybe you should start. If you go to Victoria secret they do free Bra sizing enev if you dont purchase anything.