Diminution of vision in such cases might be due to watering, discharge or some form of corneal involvement. In case of viral conjunctivitis sometimes there are subepithelial haze and deposits or an ulcer which might decrease your vision.
Next Steps
If this problem persists you must visit an ophthalmologist for slit lamp examination.
Health Tips
keep your eyes clean at all times. Don't let discharge accumulate in your eyes. Use your eye drops regularly as prescribed. As infective conjunctivitis is a communicable disease one must take precautions like using tissues, washing hands after touching eye, using separate towels to prevent others and family members from getting the infection.