What you are describing sound more like a rotator cuff injury than brachialis injury. Or it can be a long head bicep issue, or even AC joint dysfuntions. The anatomy is such that upper trapezius and brachialis are so far from eachother taht this is not a likley scenerio.
The long head of bicep tedon is situated right between the 2 tubercles where all 4 rotator cuffs are attached to. Above and medial to it is the AC joint. Trapezius is posterior to all that, and brachialis is not that superior and much deeper.
You should go see your ortho for exam. Many times a ptient would have rotator cuff tears, even complete tears, and not even know it. These patients, when confronted with the conclusive findings, would often say, "Oh, I do remember a slight twinge, it was not that a big deal...."
Go get it checked out. Rotator cuffs will always tear before any of the larger and more superficial muscle get injured (I have yet to witness a deltoid tear in my career). The problems is, IF you have a COMPLETE tear, whcih many do not realize (excpet supraspinatus, which you will not able to abdduct your shoulder in a normalized manner), there is a time period to get this srugically fiexed. The reason is muscle fibers, once detached, will coil into a ball and start to be infiltrated with fiber and fat. If left foor too long, surgeons will NOT be able to physically attach it. Rotator cuffs are deep and short so you have a few months to correct a complete tear. Longer muscles such as quads or bicept, you should get it done within a few days and at least less than a month!
Go see a ortho MD..