Is my horse sick?

[guest@endurance.net]

Susan pynevalley@aol.com
My horse was always a good drinker of water, but suddenly has begun REALLY
tanking up.  The first water we come to, even a muddy puddle, he will stop
and drink alot.  He then proceeds to fill up at every water hole.  Where I
live, there are many creeks, puddles, etc.  He used to just start drinking
after working awhile, but this behaviour is happening right at the
beginning of the ride.  I had him in a new pasture that had a creek in it
and I was worried he wasn't willing to enter the creek.  I noticed my
other two horses were not drinking unusual amounts when ridden.  I now
have all 3 back home and the Arab continues to drink large amounts of
water from the pasture bucket.  In addition, my husband and I noticed his
attitude has changed.  He normally sticks close to the two mares we own
but after moving temporarily to the pasture I mentioned above, he just
started keeping to himself.  On one ride, he was way outwalked by the
other horse and could have cared less.(this is an Arab I'm talking about)
Definitely not himself and seemed very depressed that day.  I have not
noticed this behaviour again while being ridden.  He was only in the
pasture where I began noticing these changes for one month.  He just came
home 3 days ago, so too soon to tell if these symptoms will just go away.
I have tried to find any information on excessive water drinking combined
with poopy attitude, but haven't found a thing.  Anyone out there know
anything to suggest?  Thanks for your help!