Tanks for posting your query.
Distended veins in neck can be due to many causes like Heart failure
, chest tumors, pneumothorax
or veinal obstruction. All your tests are normal and no cause is found, Now you should go for detailed investigation and get ECHO of heart to rule out any valvular disease. Sometimes, valvular heart disease
present spontaneously and other tests like ECG and xray may come normal.
Hope it helps.