Treadmill headrush?

I used a treadmill today for the first time.
When I was done and walked off the treadmill,
I felt a "headrush" -- a disorienting feeling.

Is that normal? Or does it mean that I did
something wrong?

I walk and bike ride periodically. I tend to
walk at 4 MPH on flat terrain.

On the treadmill, I ramped up to 4.3 MPH, the
speed at which I walked for most of the time.
I used the treadmill for about 30 min. That
includes 5 min of warm-up and cool-down. I did
the cool-down manually. The "cool down" button
wanted to slow the treadmill to 1.3 MPH. I
cannot walk that slow :-). I kept myself well
hydrated during the exercise. I was not hungry
at the time I exercised.