This sounds very serious. Tell me more about the red eye. Is the
whole of the eye fully reddish, encircling the whole area? From the
way you describe it, it sounds like glaucoma, but I would do well,
if you can get a picture for me.
Acute, painless, one-sided red eye suggests glaucoma, and requires
immediate ophthalmologist intervention. Diabetes can lead to
glaucoma as it is one of the complication.