Some other observations:

I usually meditate while lying down. I have felt dizzy while meditating before (similar to the spins); I've also felt like I was stuck in a sideways tilt (almost like I was lying in a hammock or a canoe that was about to tip over). This was different, though, as I was rotating fairly quickly and at a steady pace; there was a specific pattern to the rotations, if that makes sense. Also, I felt like the rotations were connected to my inhales and exhales; if it happened again, I feel that I would be able to direct the rotations. It was like I was pivoting. A final observation to note is that, at times, it felt like only my upper body was rotating; it felt distinctly separate from my lower body on several occasions.