What Causes Sharp Stabbing Pain Under The Armpit?

In the last week I have been had sharp stabbing pains under my left armpit to my elbow with very sensitive skin and sunburn sensations. Sometimes the aching feels deep and seems to travel to my left shoulder blade. There is nothing that I've done out of the ordinary. It just came on suddenly. I am a healthy 60 yr. old female..The only problems that I currently deal with is some anxiety, some lack of sleep, mild headaches occassionally, and once in awhile some dizziness..that goes away after I've eaten a banana. Any suggestions?