I'm 20. i have a sharp pain from my right breast to my armpit what does this mean?

Rib cage: Sharp,shooting pains like this often originate in the rib cage from a pinched intercostal nerve or an intercostal muscle irritation or strain. Usually the discomfort is worsened by a deep breath. Avoid further straining of the rib cage. See your Physician if fever,cough develop or if symptoms don't resolve.

May be muscle: This may be muscle. You can see your physician and see if the diagnosis is clear. You need to note what makes it better and what makes it worse. You will an examination and maybe imaging.