What Causes Numbness In Chest And Armpit?

Hi, i am hopeing someone can help me,afew days ago i noticed the back of my left arm,the inside of the same arm and going from the armpit to the middle of my chest across my left breast feels numb,what i mean by numb is ,its not totally numb but i have lost some feeling there.why i havent visited my own doctor was because i thought it would just go back to normal in aday or two,but it hasnt.I am hopeing this is not serious. Thanx Cora