What Causes Visual Hallucinations?

the other day when i was looking down at my carpet I saw a small orange spot, I thought someone has spilled something but then it started to get bigger when i reached down to touch it, it was not wet or sticky then i looked up and did not see it but when my eye moved further up the carpet the spot moved so it was my eyes seeing the spot. after blinking several times and looking up at the white ceiling when I looked back down it was gone. So what was that?