I paid close attention to how I was feeling on Sunday, and it is definitely an equilibrium problem and not light headedness. I feel fine - good, actually - throughout my walk on the treadmill, and my breathing is fine. Just before I stop the treadmill (after my cool-down) I'm breathing normally and feel perfectly fine - it's just when I actually stop the machine that I feel off. It's very much what several of you describe - like the ground under me is still moving but my body has stopped, or vice versa. I'm glad to know it's not just me, and that it's not anything to worry about! Last night, after stopping the treadmill I stayed on it for an extra 30 seconds or so, putting the iphone away, etc. I think the extra recovery helped - I didn't feel so much like I was going to fall over when I stepped back down onto the floor. Apparently I just need to take my time instead of darting out of there like I'm trying to avoid being seen.
Thanks again for all your feedback!!