What is feeling light headed and dizzy a symptom of?

I have been feeling extremely light headed and dizzy for 3 month now and I've no clue why even though my Doctor. Could this be assosciated with something serious? Ive done blood & urine check, MRI brain scan, heart scan, treadmill exercise,
eyes & ears test. No sign of virtigo & everthing was fine. Please share your information with me if anyone of you out there have the same. im only 30...

Thank you!