Red eyes are caused by enlarged, dilated blood vessels leading to the appearance of redness on the surface of the eye, and, the degree of redness or appearance of blood is not the factor that determines what is serious.
Do you feel constant, boring, pressing, burning, stabbing or piercing pain at one side of the head, mostly located behind the eye or in the temple, sometimes radiating to the neck or shoulder?
Is this a headache that usually lasts 30-45 minutes or sometimes up to 3 hours and strikes at the same time each night or morning, often at precisely the same time during the day a week later?
Have you noticed that you are agitated and restless during the attack of headache?
Do you get eye tearing together with eye redness?
Are you experiencing nasal congestion, too?