Q.Crepe Myrtle Dropping Leaves
I just planted a new crepe myrtle three weeks ago and now it is dropping its leaves. It is in a southern exposure in a well drained spot. It has had enough water. I do not see any aphids, but I dusted for them anyway. It has new growth on the end of its stems.

Certified GKH Gardening Expert
It sounds like the tree may have transplant shock. This article will help:
https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm