The front part of the upper arm is composed of a network of muscles, the smallest of these being coracobrachialis. Even though he is small, he can cause some pretty mighty issues.
The primary function of this muscle is to help bring the arm towards the body. A secondary duty is to help stabilize your arm when it is at your side. The coracobrachialis muscles lies midway up and on your inner part of the humerus. Along with all it's own responsibilities, it is also a synergist to the anterior deltoid and biceps in flexing the arm at the shoulder.
How do I know if I need bodywork done of this guy?
The pain will typically show up in the front part of the shoulder, and can even present itself in the back of the forearm/upper arm. Depending on how long you have been dealing with pain, you may have a shooting pain that feels more acute going down the arm. If put off for too long, you may experience loss of range of motion (raising your arm overhead/putting your hand behind your back).
Another very distinct referral is pain into your middle finger, specifically in extension.
Most cause of injuries to the coracobrachialis are physical/sports related: push-ups, climbing, throwing, or weightlifting. Most of the time, strains like these will go away on their own, with a little rest and recovery. Try this stretch out to target the coracobrachialis