I have numbness, tingling and sometimes pain down the outer side of my left leg frequently when sitting or lying down. what could be causing this? should i visit a neurologist ?

Not a rare thing: Compression of the lateral femoral cutaneous nerve, usually at the groin can do what you describe. Common causes include tight clothing compressing the groin, intrapelvic disorders, aneurysms of femoral artery, pelvic brim bone spurs, etc, etc.) neurology consult will help make diagnosis & maybe resolve it too.

If persistent: Yes peripheral neuropathy or radiculopathy can have many causes. See a neurologist.