My left breast feels like i've been punched in it. i went to my doctor and they didn't find any problems. why does it hurt so bad?

Could be hormones: Or it could be that you hurt it like running in to something when you did not even realize it. If since it feels like a punch and not a pinpoint pain it is not as worrisome especially in a young girl your age. If it does not get better in a couple of weeks i would go back to the doctor again. An ultrasound might be warranted anyway.

Pulled muscle: Assuming you have been adequately evaluated for more life-threatening conditions, my best diagnostic guess is that you have a pulled muscle between the ribs beneath your breast tissue. It can be very painful. Another possibility is a pinched nerve in the thoracic spine with radiation around to the front. Best wishes to you. Please follow-up persistently until it goes away or is diagnosed.