I agree with Shelley, TOS is often diagx as RSD as there are cross symptoms.
I say that one often goes with the other.

Do you have problems holding the phone to talk, typing, driving, blowdrying your hair?

When the arms are raised it can compress the nerve bundle of Cervical nerves, C5/6/7/8 and T1 as they travel behind the collar bone and in front of the first rib. press the area behind the collar bone about midway to the shoulder, is it tender?

The compression of the brachial nerves Thoracic outlet TOS, can cause heavy arms, weak arms, hands to fall asleep, like waking with the hands asleep or ring and pinkie finger numbness or tingling. Don;t sleep with the arms overhead either....

A TEST: Bring your arms up and back like the stick um up position of being robbed. all the way back until you feelit in the shoulders wing and spine...if your round forward or do not hold them back to close the outlet it will not reproduce the symptoms. SO mot just raise them up...bring them back as putting the wings together...Generally we say open and clost the finger for three minutes, but many cannot do that, put them down when it is causing problems.

If you try to checkl your posture from rounding shoulder you may be able to improve this fatique. Place a campells soup can in a nylon back pack to encouage the shoulders back....

If someone looks at yhour profile do you have rounded shoulders and a swan like or turtle neck from reverse curve?
Hope this improves for you
Dianne