I was sitting there reading a book when I started to notice a blurred spot in the middle of my vision. I tried to ignore it, but it was getting more noticeable as the moments passed. After about 30 seconds to a minute I couldn't read because it was distorting the words.
Needless to say, it was a little disturbing. Especially because it kept getting bigger. Not only could I not read, I was having trouble walking around the room. When it gradually started to move to the side, but it was still getting larger, I decided to google these symptoms.
After a bit of searching I ran across this article by the Mayo Clinic that described the experience perfectly. I was experiencing either a Migraine Aura (or possibly an ocular migraine). They said it was harmless and would go away on its own. Eventually it did, but it left a headache that was settled nicely behind my eyes.
I'm feeling fine this morning, but it defeated the majority of my sermon prep last night.