First of all, there are all sorts of visual effects that are caused in the eye itself (e.g., 'floaters') and not obviously linked to neural diseases. Actual hallucinations (let's not beat around the bush) can be caused by all kinds of brain/neural issues.
In June I went from a few weird peripheral distortions one day to homonymous hemianopia (http://en.wikipedia.org/wiki/Homonymous_hemianopsia
) in less than 2 weeks. I and my primary both assumed it would be MS-caused ON, but the RN at my MS clinic said it wasn't ON, and it took an ophthalmologist and another brain scan to call determine it was a stroke (and fortunately my vision returned to normal within a month).
So you'll be wanting to get that checked out ...
Best of luck,