Could my diabetes cause one of my eyes to be red and painful?

Diabetes can cause eye problems. Pain or pressure in one or both of your eyes could be a sign of damage to your retinas. Blurry or double vision, dark spots or blank areas and trouble seeing out of the corners of your eyes are other symptoms to watch for. The high blood sugar associated with diabetes, sometimes coupled with high blood pressure, can cause the tiny blood vessels of the eyes to swell and weaken, and possibly leak blood into the vitreous (the gel-like fluid within the eye). This can keep light from reaching the retina. Those damaged blood vessels may also develop scar tissue that could eventually pull the retina away from the back of the eye, a potentially dangerous problem called retinal detachment. You can minimize your risks of eye problems by keeping your diabetes under control. Getting regular eye exams can help you catch problems before they become too serious. Report any unusual eye symptoms to your doctor.

It is more common to have red, painful eyes from a viral infection or allergic reaction. Allergies to pollens and dust in the air are the most common cause of red eyes, but this rarely causes pain. An eye infection that can cause red eyes is viral conjunctivitis, or "pink eye." Unfortunately, this infection has to run its course because antibiotics cannot help speed recovery.
Serious bacterial infections can start on the surface or behind the eye of a person with diabetes and require strong antibiotics to cure. A common complaint of patients with either viral or bacterial infections is that they wake up in the morning with their eyelashes sticking together from the pus that has collected over the night. If your pain is more like a pressure sensation, then you may have glaucoma (although most glaucoma is painless).
Glaucoma is too much pressure in the eye and is more common in people with diabetes. It can be detected during your yearly eye exam. The test involves blowing a small puff of air (which doesn't hurt) at the surface of the eye. Your doctor may prescribe eye drops that lower the pressure in the eye. This condition is definitely worth finding early because it is treatable. Left untreated, glaucoma can result in blindness.