Eagle, there are so many things that it could be. It sounds inflammatory. Can you feel your chest wall beneath your breast? As you said this could be muscular. Does it hurt when you move your arm in various positions? Just trying to see if its the breast itself or muscular. I was plagued with costochondritis in the 80's and 90's particularly. It did indeed feel like a heart attack. I would find I could inflate my chest with a breath just so far before a very painful"catch." Also it would ease slightly if I sat forward. That is chest wall pain. I only got relief from prednisone, unfortunately. Just throwing all these thoughts out to you.
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