Shoulder feels like a knife is cutting it every time I lift my arm?

The pain is in between where you can see the cut between the front part of the deltoid to the side part. It only hurts when I lift my arm up but it feels like as if something was stabbing me. I havent worked out in a while but the pain is starting to worry me because it hurts a lot when I try to lift something and i'm leaving to the Marines soon and I don't want to be discharged because of this.