Question

can bad eyesight cause headaches and nausea

Answer 1

If someone had weak eyesight that does not mean that it has headache problem.

Answer 2

Eye strain often causes lymph constriction. Headaches and migranes can be associated with lymph constriction and blockage. People who strain their vision system during heavy movement are very prone to motion sickness. The lymph system drains near the heart and near the colon. They are all connected. 

People who learn about natural eyesight improvement not only improve their eyesight but also dramatically reduce their tendency for motion sickness.