Feelings of lightheaded news/dizziness off and on for 12 hours, no other symptoms. no chance of pregnancy, routine blood work 3 months ago normal.

Lightheadedness: If it does not improve soon, go to the emergency room as there are many reasons for this. They will make sure there are no life threatening issues like irregular heart beats such as supraventricular tachycardia which is most common in your age group. Other conditions like dehydration and inner ear infections are benign and self limited.