To be honest i'm somewhat concerned, you are now asking about ON after only last week being anxiously concerned about you having Nystagmus, please understand that continuing to seek MS related symptom information is likely causing your mental health more harm than good......
Optic Neuritis (ON) is inflammation of the optic nerve, it is most commonly unilateral and causes blurred, grey and dim vision, symptoms can range from mild to severe, with the 3 main symptoms suggestive-consistent with ON being pain with eye movement, loss of visual acuity 'and or' visual field, and loss of perception of the colour red.
People typically describe experiencing a sudden or gradual decrease in vision in the effected eye over a few days, descriptions like significantly blurred vision, like looking through a smudge of Vaseline, large area grey - black, complete blindness in the effected eye. Vision loss lasts on average 7-10 days and then gradually begins to improve between 1-3 months. Permanent damage to the optic nerve occurs in about 85 per cent of cases, but damage to the optic nerve does not always cause serious permanent visual problems, ON is also typically short-lived and resolves by itself without treatment in around four to 12 weeks.
Pain with eye movement, descriptions varies greatly, pain in or around the eye, dull pain like pushing on a bruise, throbbing, pressure, intense like an ice pick through the eyeball, pain typically resolves over a few days even though vision is still effected. Loss of colour brightness in the effected eye, reds predominantly wash out but then there are also greens too.
It's pretty common to additionally experience Uhthoff's phenomenon, which is caused by an increase in core temperature temporarily worsening vision and other MS issues, so things like hot weather, baths, sauna's, menstruation, infections etc etc ON is not exclusive to MS it is also associated with autoimmune conditions and bacterial or viral infections.
You ask what this could be, what it could be is more likely nothing meaningful towards the health of your optic nerve because what your describing is very vague, your basically saying this is a small discomfort that hasn't effected your vision at all and is inconsistently happening.....that type of description literally could be nothing or if any, something along the lines of eye strain, stress, lack of sleep, dry eye, ocular muscle strain etc etc