the leaves on my crepe myrtle are turning red
This sounds like Cercospora leaf spot. It can be caused by damp weather or excessive rains or watering.
Rake and remove all infected leaves that have dropped.
Long term this is not detrimental to the Crepe Myrtle.
By the time you see the issue with the leaves, it's really to late to treat the fungus.
Fungicides are not effective on treating leaf spot.