What could be causing one bloodshot eye?

Some thoughts: Couple common problems can do this: corneal abrasion (scratched eye)-perhaps something got into that eye. Subconjunctival hemorrhage-when a tiny blood vessel beneath the surface of the eye breaks (not always assoc with a known injury). It's usually painless and resolves w/in 2 wks w/o specific tx. If you have other bleeding/bruising problems, high bp, vision loss/changes, significant pain see doc.