The only time I ever had eye pain was when I had Optic Neuritis. But my eye wasn't red. If I were you I'd let the doctor take a look at it. Sounds like it might be some type of infection. If it was related to the coumadin it seems like it would be in both eyes.......but I'm no doctor so I'm just guessing. I'd let a doctor have a look at it. Hope you feel better soon!