If you actually see a demarcated area of bright red blood in the white part of your eye, then this is likely a subconjunctival hematoma. This occurs when a small blood vessel under the surface of the white part of your eye bursts, allowing blood to escape. This most commonly occurs with coughing or straining, but it can also occur accidentally, for example by bumping or scraping your eye. Subconjunctival hemorrhages, although they look ugly, rarely require medical treatment, as they get better on their own over a period of days to weeks.
Second, if the area of 'blood' is actually more like a general redness or irritation, then this could be conjunctivitis, inflammation of the white part of the eye, either from an infection or from allergies. To rule out the possibility that you have conjunctivitis, you should see your doctor
who will be able to recommend an effective treatment.
Furthermore, if you have any concerning symptoms, such as severe eye pain, blurry vision, or headache
, these might be signs of a more serious problem and you should seek immediate medical help.