Webangel - my first thought was like SaranBear's - I live in central Texas and the heat will zap my VERY fit dogs QUICK when exerting energy like trotting after a bicycle. We walk briskly every evening and we have to wait until almost dark, or the boys (2.5 and 5.5 yrs old) will lag behind before the end of our routine walk because of the heat. Don't push her too hard if heat is the reason - it can go from ok to serious very quickly if heat is causing her to hold back. I agree on a vet check to insure nothing medical is going on.....but as the temps change in the next month or two, I know my guys will perk back up and we'll be adding distance to our walks because their stamina will be back to normal
Good luck - I hope it's nothing serious with her and just plain old "too darn hot" in her book for this nonsense
