Bloodshot eyes are usually caused by eyes drying out and not getting enough natural tears to keep the eyes lubricated naturally.
When your eyes are not lubricated properly and dry out the eyes become irritated and red which leads to bloodshot eyes.
Usually lubricating your eyes when they're bloodshot with lubricating eye drops can help get rid of bloodshot eyes.
Changes in your bodies hormones can also cause your eyes to become bloodshot as well.