What Causes Constant Lightheadednes?

Hi ive had lightheadedness for the past 4 months everyday I've had an MRI scan on brain,vestibular tests,echo cardiogram and blood tests and I've been given the all clear.I've been prescribed cymbalta as my doctor thinks it is anxiety. I've been on cymbalta 30mg for 2 weeks and still have the sense of being off balance and lightheaded constantly. Although i find the symptoms are now more bearable as prior I could not function.. Can anyone explain to me what this could be?