Seems like it would be your subclavius, it pretty much runs right along the area you are describing your pain to be in. You may have pulled it doing all that close grip work, it does work to pull the clavicle down and forward. If you pulled it, it might just be semi-healed now, but in a shortened state with a bunch of adhesions that make it painful to try to stretch the muscle. Which would explain why being hunched over and internally rotated relieves your pain. I would try soft tissue work specifically right under the clavicle. I doubt a ball will work since it's right underneath the clavicle, probably need to use fingers. See if it feels tight/painful to the touch. After a few sessions trying to loosen it up you might feel more comfortable trying to stretch the muscle without pain. After that you can start to add in some rehab exercises which can start to strengthen it.
Soft tissue work on the surrounding muscles isn't bad, but I think if you get right on the source of the pain it tends to work a lot better. I've had a few injuries where the pain just lingers for months until I get right on whichever muscle is the source of it and massage it, then my symptoms resolve rapidly.