“Why was one eye bloodshot when I woke up?”

A red eye or red eyes should be evaluated by an eye doctor, personally because there may be many reasons for the redness. Your medical history and any medications you use needs to be determined before any treatment is begun or before the cause of the red eye can be evaluated. Red eyes come from eye rubbing, from the presence of a foreign body, from a variety of conjunctivitis problems including allergic, viral, and bacterial, there are numerous medical and eye conditions that can cause a red eye or red eyes, air-borne irritants can cause red eyes, sun exposure and dry eyes can cause the eyes to be red, corneal abrasions and turned eyelashes can cause an eye to be red; so there may be many causes and many reasons. And the treatments for all of the above are varied and different. I would strongly suggest that you see an eye doctor to have this issue properly evaluated.
Good luck and best wishes.

Go in to your optometrist for a checkup and dilation

Sometimes the tiny blood vessels in your eye can break for unknown reasons and cause your eyes to be bloodshot. It usually takes about 1 or 2 weeks for the blood in your eye to drain back into your body. The medical term for this condition is called a "subconjunctival hemorrhage" and it is usually harmless and doesn't affect your vision.

Breakage of capillaries