I'm a 19 year old female, and when i take a deep breath, i feel pain in the middle of my chest. it's happened before, what could it be?

I would doubt a cardiac or pulmonary cause of this symptom in your case. Most likely the discomfort emanates from the rib cage caused by mild irritation/inflammation of a joint at the junction of a rib&breastbone. If so, it may also be tender. Another possibility is a pinched intercostal nerve that runs between 2 ribs. These issues usually resolve on their own. If not, see your MD for evaluation.