Optic neuritis is an inflammation of the optic nerve. When a doctor examines your eyes, he can see the optic nerve at the back of each eye. If you've had (or have) optic neuritis, the nerve is pale in color (paler than the other eye), and sometimes swollen in appearance.
Optic neuritis can cause pain in your eye, but not always. It can also cause problems with vision: loss of color vision, blurred vision, sometimes double vision, sometimes a sort of "halo" will form around objects you're trying to see.
When you ask, "can it fluctuate throughout the day", I'm not sure I understand what you really are asking. Optic neuritis is a condition. You either have it or you don't. Optic neuritis doesn't "fluctuate", but it can cause vision problems which can fluctuate during the day..
but lots of other things can cause vision problems too, like migraines, simply bad vision, etc.
Why don't you describe what's happening with your vision, and we'll try to help sort it out.
Final thought: I'm a real wimp about my eyes. If ANYTHING weird goes on with my eyes, I'm off to an opthamologist right away. (Opthamologist, though..not an optician, not an optometrist, but an opthamologist, who has major medical training and specialized in eye disease, not just vision problems...) I'd suggest you do that, too.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....