It's a subconjunctival bleed
It can occur after a sudden or severe sneeze or cough, heavy lifting, straining, vomiting , constipation or even rubbing one's eyes too roughly.
Sometimes due to sudden shoot up of blood pressure can lead to this bleeding.
It is self resolving.
Usually it disappears within two weeks..
Warning symptoms should be ruled out like - diminished vision, floaters means floating mosquito like black spots in front of eye or pain.
Next Steps
Blood pressure should be checked once.
Health Tips
Avoid rubbing of eyes
Avoid strains.
Avoid weight lifting.
Treat underlying causes like constipation or coughing.