My crape myrtle is the biggest drama queen when it comes to water. Maybe not the thirstiest of my trees, but the one most likely to swoon if it goes a bit long between waterings. When I'm watering my trees and its being dramatic, if I water it first, before I'm finished watering all my trees it has noticeably perked up.
All that said, if you're watering sufficiently, and the tree is doing this, you've got a root issue. They move water pretty quickly so it could be as simple as watering more often. I'd also consider a bit more shade and maybe a layer of sphagnum moss ...and make sure that this one gets a thorough examining in spring time when its safe to repot. (Another note, my crape myrtle is one of the last to leaf out particularly after years where it has been weak heading into fall, ...so you may think its dead next spring but don't toss it.)
