How do your eyes become bloodshot?

Irritation: The front of the eye is well vascularized with fairly small vessels. These are very responsive to irritation, allergy, any type of eye illness or trauma and will respond by dilating and causing the eyes to look red. This will disappear with a time span depending upon the cause.

Increased blood flow: Can be brought on by simple eye strain irritation from allergens and infection. May be worth while seeing eye doc for consultation.