What Causes Redness In The Eyes?

One of my eyes suddenly turned red. Not the entire eye, about one-third in a semi circle toward the inside, and the bloodshot appearance in the affected area is significant. I first noticed it when I woke up this morning. No pain or swelling, and I did not suffer an injury. My vision has not been affected, at least not at this juncture. For a few days my eyes have felt itchy and I attributed it to pollen/allergy. Yesterday I purchased Visine eye drops for Allergy Relief. I used them three times today. Some temporary relief, especially for the healthy eye, but not much help for the problematic one. There has been no noticeable change in the amount of redness since I first detected it this morning. Can you help? Thanks