What could cause one bloodshot eye with no pain?

Many things: If it actually looks like a small amt of blood, could be a subconjunctival hemorrhage, which is common, painless and doesn't affect vision. Other causes of red eyes (of which there are many) can have an asymmetric presentation, anything from pink eye, blepharitis, dry eye, allergies, hsv conjunctivitis, etc. Mild cases of these things may not cause pain.