The term "red eye" is used to describe eyes that are red and bloodshot. They develop when blood vessels near the surface of the eye become swollen. The most common causes of red eyes are painless but serious conditions can be quite painful.You can develop bloodshot eyes temporarily after drinking alcohol, staring at a computer screen too long, or forgetting to change your contact lenses. But bloodshot eyes can be caused by something more serious as well, such as an eye injury or infection.
Source: Verywell Health