If your crape myrtle's leaves are curling it is typically caused by lawn chemicals, particularly phenoxy herbicide. These chemicals are frequently sprayed on lawns to control broadleaf weeds. If over time too many small amounts of this herbicide get on your plant, it could cause your plant's leaves to curl.
This problem could also be caused by an infestation of an insect called aphids. These tiny creatures suck sap from leaves and stems of crape myrtles, causing their leaves to turn yellow and curl. Keep an eye on your plant's foliage for any discoloration and always check for any insect infestations to ensure they are caught early.