Thanks for writing to the forum!
The pain can be due to nerve compression or due to kidney stones. Compression of spinal nerves in the lumbar region can cause low backache with pain referred to testicles and so can kidney stones. Pain in testicle could also be due to infection, like prostatitis, epididymitis or orchitis, varicocele or spermatocele. Get a culture sensitivity test for both semen and urine done. You may need a MRI of lumbar spine region, ultrasound of abdomen and testicles.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!