Can contraction of scm cause temporary bulging in external jugular vein? bulge is painless, soft, <1 cm. no bulge when muscle is relaxed.

Not normally: No, not normally. Contracture of the SCM would not really affect the jugular vein unless you contract it to the extent that increases the pressure in the venous system which causes venous distension in general. You may have a sift tissue growth superficial to the SCM that then becomes more prominent as the muscle contracts. Don't hesitate to be seen for it.