I've been having red veins in my eye for months. what can it be?

Vision is priceless: Simple rubbing of your eye can cause some redness, but often resolves quickly. Allergies can cause redness, so does infection etc..Persistent redness/prominent vessels wtih symptoms--pain/discomfort, vision trouble etc...Should be checked by doc. If this something new, especially if only on one side, consult doc for eval. Don't ignore this--vision is priceless. Good luck.