Why do i have pain in my left shoulder and under my left breast?

Pulled muscle: Its likely musculoskeletal at your age. Check bra tightness, try 2 aleve 2x per day for a week if no hx of ulcers. If nausea, vomiting, etc ensue go get seen, low probability for cardiac but god forbid i give the wrong advise on chest pain. Zantac (ranitidine) may help if it's reflux.

Need more info.: 95% of diagnoses require merely a careful history. For your pain, it would help to know the type and severity of the pain, whether or not it raditates to some other location, whether it occurs intermittently or continuously, how long you've had it, whether you were overusing your arm and shoulder, is it painful to touch the area, and the effect of breathing and exercise on the pain.