I have been feeling lightheaded on and off for over a year now. i have seen a couple doctors, and they never seem to find a cause. what could it be?

See your physician: These symptoms may represent orthostatic hypotension, a headache or other neurological condition (eg BPPV), sinus issues, vasovagal response, or a vascular/blood flow issue. You should keep a log of the location, timing, duration, and intensity of these episodes and take the log with you to your physician. Avoid sudden head turns. http://www.nlm.nih.gov/medlineplus/ency/article/001420.htm