Both are common conditions and the association of the two is often coincidental or by chance. When related the causes are legion and too extensive for listing. It's like saying "what causes muscle cramps and a rash on the face.
JCH MD
Could you please explain the cause for simultaneously having Red Eyes with a Headache ?
See an ophthalmologist for a medical eye exam and a update of your glasses RX. Find one near you at www.aao.org
JCH MD