Get specifics about the injury from your client and inquire about the other activities they have been engaging in.
I always remind my bootcampers and clients that active people have more aches and pains because they are active!
Keep your certifications current along with your liablilty insurance.
Make sure you have release forms signed by all of your clients prior to working with them and let them know that they will be sore and can expect it.
It sounds to me that the client did not mention anything during the session and the claim of having been injured came out of the blue. I hope that you keep meticulous track of the exercises you do with the client and can check what you did. Did you do any new exercises or increase the weight on existing ones? Have you had the client for a long time and know of any problems that existed before? Is it a brand-new client? Check the medical history and your assessment.
Ask as many questions as you can about it. Many clients are very ready to blame exercise for any pain/ discomfort / injury when the incidence (if there was one) occurred during gardening or other activities.
What is the client asking you to do? Until you know more, show concern but gather your facts. If you have acted consistent with your scope of practice and proper precaution, there is nothing to fear.