If you've not suffered a knock, then infection is likely but it's very difficult to determine without analysis of the tears from that eye. If no improvement in 3 days, you should contact the/a doctor again.
It could be a broken blood vessel--I woke up with one that looked horrible but no pain. Opthamologist said just alternate moist heat/cold packs. If it was an infection, I think there would be a crusty discharge or swelling.