Crape myrtles are deciduous and you can expect it to begin shedding leaves soon. Do NOT over water this plant at any time, but certainly not at this time of year when it is slowing down for the season. I hope you haven't fertilized it.

Look for crape myrtles to bud and leaf out in the mid- to late spring, always a bit later than everything else. They flower in the summer.

Crape Myrtles are in fact deciduous and will grow as much as 4' per year with the first year always being the slowest. They are very drought tolerant so it may never be necessary to water the plant. Here, they set leave in early spring but do not produce flowers until mid to late summer. I bet yours, as long as its not sitting in a pan of water, will begin to show abundant growth next spring.