I have a sharp pain going from my left armpit to my left boob, nothen has happened to cause this. What could this be?

Likely OK. It sounds muscular in nature, like you strained a muscle from your armpit or chest--which can cause that kind if discomfort. If the symptom worsens or persists or if you feel a mass, be sure to get a medical examination.
Several things. Pain that severe needs to be worked up by your doctor in person. He/she will listen to you, perform an examination and possibly order some tests to see what's wrong with you.