I'm 14, what could cause left breast pain and left arm pain?

toi much exercise or your boobs are growing

Pinched back muscles often send the pains around your body on the nerves to be pains in the rib, chest or heart areas. The nerves your pains took ended up in your breasts instead. The pain in your arm can be from the nerve getting trapped by the back muscles to send the pain down it. to get rid of the pains you have to free up your back muscles to release the muscles and here's how to free them up:

Back:

(do while sitting)

Place your left hand on your left leg next to your body. Place your right hand over your left shoulder, fingers over the back and the palm in the front and firmly pull down on them and hold. After 30 seconds slowly lower your body forward and to the outside of your left leg, keeping your left arm fairly straight as you do. When you reach your lap remain there for another 10 seconds, release the pressure but rest there for another 30 seconds. Then reverse your hand positions and do your right side.

For best results relax your body first by taking a deep breath and exhaling then remain this relaxed.

if there is also swelling going on or your feel heat chances are you got an infection going on, something like an abscess.

if it is one the MD will open it up, give u antibiotics and it will be treated in a wound care clinic.

so do your stretches and watch it.