Red eye might be a symptom of different processes:

• Conjunctivitis: the blood vessels of the conjunctiva –the clear membrane covering the white part of the eyeball– might become inflamed and give the eye a reddish appearance for several reasons: infection (virus or bacteria), allergies (pollen, especially in the spring, or other allergens such as mites) or irritation (cosmetics, chlorine from swimming pools, etc.)
• As well as conjunctivitis, other inflammatory processes that might also lead to red eye are keratitis (corneal inflammation) or uveitis (inflammation of the uvea, the membrane that envelops the interior of the eye and the tissue with most blood vessels in the body).
• Dry eye: because either the tear quantity or quality is insufficient, dry eye means that the surface of the eye is not well lubricated and, therefore, can become inflamed and irritated, and then red. This problem of multiple sources and different degrees of severity affects around 20% of the population, as a result of hormone changes, environmental factors, systemic diseases, other ocular pathologies, etc.
• Incorrect use of contact lenses: soaking lenses for too long, sleeping with them in or poor hygiene when handling them and storing them all lead to the appearance of irritation/infection and, therefore, red eye. You should not swim while wearing contact lenses, due to the presence of micro-organisms in the water such as Acanthamoeba, which could severely infect the eye and produce a corneal ulcer.
• Red eye is one of the characteristic signs of corneal ulcers, lesions that threaten the eyesight if not treated in time and that are generally due to infection.
• Severe attack of glaucoma: although glaucoma is often symptomless until advanced stages of the disease, one of the signs that often accompany a severe attack in patients with a narrow ocular angle is red eye. This is due to the blocking of this angle through which the fluid bathing the inside of the eyeball drains, so its closure leads to a sudden increase in intra-ocular pressure.
• Endophthalmitis: infection of the entire eyeball is one of the most severe causes of red eye, as it jeopardises the well-being of both the eye and eyesight. Where it is exogenous, the infectious agent might enter through an open trauma or intra-ocular surgery (this occurs in less than 1 in every 1,000 surgeries), whereas if it is endogenous, the infection invades the eye through the patient’s blood stream.
• Other possible causes of red eye are different types of trauma (from foreign bodies to corneal erosions, chemical burns or ocular perforations), and the rejection of a corneal transplant.

Source: https://www.imo.es/en/symptom/red-eye/