What Causes Frequent Dizzy Spells?

Hello. Periodically I experience dizziness. It can happen when I'm sitting down, standing, concentrating on something, talking to someone, etc. The room will literally spin for about 4 sec and then it goes away. I typically do not feel nauseous but do experience it from time to time. I am not taking any medicines besides a multi vitamin. I've had blood work done about 6 months ago and everything came back normal. I also did not have vertigo. Any ideas/suggestions?