as a swimmer myself there can be a tendency for hyper-extension of the lumbar spine predisposing to Wikipedia reference-linkspondylolisthesis if the core strength isn't sufficient and depending on stroke and technique (female butterfly swimmers are the most at risk). however red flags do fly with the patient's age, bilateral symptoms and onset.
i am very interested in Wikipedia reference-linkMRI results but would also like to see lumbar neutral, flexion and extension standing plain xrays to determine if there is a dynamic spondylo component (ie not evident in neutral, exaggerated in full extension and reduced in full flexion).

keep us informed
cheers,
msk 101