Injury to the eye
A minor eye injury caused by something that has poked the surface of your eye or you rubbing your eye a bit too hard or an insect flying into the eye. These may rupture the fragile blood vessels below the sclera, causing blood leakage. Subsequently, it might lead to a red spot on the eye. It is usually advised that you wear protective eyewear like eye shades for sports and activities that involve flying debris. Sunshades are great wear to protect your eyes when you are outside, reducing the risk of small dust particles flying into your eyes along with the wind.
Irritation from contact lens
A small dust particle caught under your contact lens may cause enough irritation for you to rub your eye hard. This might cause blood in the capillaries under the sclera to break or leak, leading to a subconjunctival hemorrhage. Make sure that you remove your lens and clean it thoroughly if it makes your eye itch. Not just that, also keep in mind not to wear your lens longer than the recommended time limit. In case you use disposable lenses, change them as per instructions on the manual.
Drugs that specialise in thinning the blood increase the chances of bleeding. Some of these medications are aspirin, heparin, enoxaparin, apixaban, enoxaparin, rivaroxaban, warfarin, and dabigatran.
Episcleritis is a severe inflammatory disorder of the episclera, the narrow tissue between the white sclera and the conjunctiva. The episclera has a thin network of blood vessels. This condition usually gets cured by itself, although some cases may lead to inflammatory disorders in other parts of the body.