Horse "shut down"

["Tamara Putman" <nis75p06@mindspring.com>]

Help!  I've been gradually conditioning Wildfire for his first 25.  We've
been going along rather nicely until last night.  I headed off down the road
and after about 30 minutes of alternating walks and canters, he just shut
down and didn't want to do any more.  He cantered when I asked, but it was
this slow lope that would have done a pleasure horse proud but he wouldn't
pick up the pace.  No abnormal heart sounds (I use a stethoscope as a HRM is
still out of my price range for now).  No abnormal gut sounds.  No lameness.
I hand-walked him about 10 minutes heading back home and rode the rest of
the way.  He finished cooling out fine, no sore back or anything.  Tonight I
lunged him and he was still sluggish at a trot, not wanting to move at his
normal paces.  He cleaned up both feed and hay, drinks fine.  Bright eyed
and alert.  In short nothing is wrong, except he doesn't want to move out
anymore.  Any ideas?

-Tamara