Why do I randomly see illusions while sober?

Sometimes when I am just sitting around doing nothing I will stare at something, usually a pattern, and it will start to swirl around or bubble or breath almost like it was influenced by a hallucinogenic drug. Sometimes I find that the whole room is swirling around like it's in space. You could say I just have a very large imagination, but I know it's not that when I realize I can not stop the strange illusions. It is not because of any drug, I do not do drugs.