It could well be that you have fibrocystic change, which is common in younger women, and especially women with dense breasts. It can be marked by breast pain. I'm also wondering if you've had cardiac testing, since this is on your left side and you mention shortness of breath; this is different than a chest X-ray.

I'd suggest you ask your doctor if this might be a heart issue; and also ask for a referral to a breast specialist, who could no doubt shed more light on this situation. We know our own bodies; and if you feel there's something wrong, you need to get to the bottom of the situation, to set your mind at ease and get whatever treatment's necessary. Good luck - PJH