photonick,
so I had the same exact problem as you a couple months ago. All of a sudden I was completely dizzy for no reason at all. I saw all sorts of doctors; primary care, ear nose and throat, neurologists, etc. They did all sorts of tests on me including an MRI and found absolutely nothing. However, by the time I scheduled all these tests, etc., the feeling got milder and milder. A month later, the feeling was barely noticeable and eventually went away.
All I can say is that it is quite possible you got some kind of virus that got into your head and messed with your equilibrium.
I have a question though. When you are standing up, do you lose your balance very easily? If the room is spinning when you are standing still, you may have a case of vertigo, but if your balance is off and you have lightheadedness, I am guessing that will go away after some time.
Hope that helps.