What Causes Severe Pain In Bicep And Underarm?

I work as a CNA at an assisted living home, I have been there 4 months, and for the past 2 months I have been having severe pain in my left bicept and under arm, it makes the rest of my arm go weak because of the pain to the point of where if I am holding something in that hand it will drop. The pain is pretty bad, goes away for a minute or two and then just comes back again. This goes on and on and keeps me up at night so just wondering on what it may be.