If it is only in one eye it could be that something had been nailed to him, like a blade of hay, or that it was a conjunctivitis. If it is in both it could be a symptom of respiratory infection. In both cases it is necessary to take it to the veterinarian. If it is something that has been nailed to him the veterinarian will do a test to see it in more detail and he will prescribe an antibiotic to him to put him during a week, just as if it is a conjunctivitis. If it is a respiratory infection, it will also give you antibiotics or timely medication. As a complement to what the veterinarian prescribes you can also wash your eyes with chamomile but it must warm, almost cold.