What Causes Dropping Feeling Followed By Dizziness?

Three times now, I've just been sitting down, not moving, and I got this sudden feeling that I'm dropping. It feels like I've tumbled off a bed, or am in an elevator whose cord just snapped and now I'm plummeting to the grou d. It's scary. After each time, I've felt a little dizzy for about 2 minutes later, but then I'm fine... How concerned should I be?