Bloodshot eye? advice please?

hope i get get a bit of advise. i occasionly would suffer from bloodshot eyes that would clear within a few days this would happen every other month or so. but 2 months ago one of my eyes become bloodshot and hasnt cleared since. it dont hurt or itch , ive had no injury and i only notice it when people bring it up what could be causing it? i havent been to a doctor as i have read loads and it always says unless its painfull its harmless