FHP - forward head posture is a no no. It will worsen the strain on upper traps.

Have your posture examined. Since you were physically fit and don't have a history of trauma, thats a good thing.

For venous/arterial, don't they do doppler scans? Jomar would know more.

It could just be plain and simple muscle imbalances. Weak shoulder stabilizers, and overactive upper traps, overactive pec minor and lats (both go together). Please have your posture assessed by a good PT, since many folks with TOS tend to have bad kyphosis (upper back, worsening/weakening shoulder blade stabilizers) and forward head or either flat or arched lower back.

It might just mean some gradual PT and return to good health.

In short, please be positive.