I often get this feeling when I am in a tall building, especially at the UW-Eau Claire campus. The library was particularly bad for this. I'd be doing homework -or more likely- casually reading, and the floor would suddenly start to move around underneath me. I'd feel disorientated, and a little nauseous. After a few seconds, or a few pounds of pressure on my head, my vertigous senses would return to equilibrium and the effect would pass. I don't know what caused this-possibly the building bending in the wind, or an odd architectual design, or what-have-you. Of course, the problem could be internal-I've never claimed to have perfect balance, or even good balance for that matter.
From a physics standpoint, our bodies are not equipped to detect motion-only acceleration. Thus, if I am "feeling like I'm moving when I'm really sitting still", there are three explanations. A.) You are accelerating-either changing direction, changing speed, or both. Check your surroundings-are they moving? Would there be a source of the acceleration around you? B.) Your internal sensors think you are moving-have you been spinning? Are you sick? Have you had your inner ear moved/removed? C.) You are crazy. Go to the nearest mental health facility and explain the situation. They will help you. They are nice people. (Note: this is ONLY for cases for which there is no other cure).