What Causes Dizziness , Nausea And Headaches?

I have been having bouts of dizziness, nausea, and constant headaches for close to two months now. I've had blood tests for anemia and hypothyroidism, abalance test to check my inner ear and all are fine. Blood pressure is a little high but the doctors don't think that's it. I'm a 30 yr old female with no serious medical history.