I went through a few years of this

I was in my late 50s and experienced sciatica symptoms (left leg pain) whenever I ran or cycled vigorously. It did not bother me really when swimming or walking or in day to day life. I went to pysiatrist who ended up diagnosing a lumbar issue sent me to PT for "Mckenzie" exercises. Unfortunately this did not clear the issue, although I do believe it can be effective for some sciatica problems. I went to a orthopedic surgeon who specialized in backs. After reviewing the scans he said he would not recommend surgery. Both the pysiatrist and ortho encouraged me to stay moving and active to the best of my ability. I'm an engineer, so I am used to problems having an identifiable cause and a path to a solution. So it was pretty frustrating not to get a more confident path forward. Nevertheless, I took their advice and stayed as active as I could. After about 3 years it went away and 10 years later it has not returned. I was able to return fully to running and cycling a little older but no worse for the layoff otherwise.

What I learned from all this is be patient and stay as active as you can. Musculoskelatal problems can improve with time. I hope in your case they can find a clear cause and help you get quickly over it.