I'm 23 y old&have left testicular pain , started a year ago, used to come with running but now sometimes @ rest . tenderness &radiate to the back?

Testis pain: The left testis is susceptible to america seal. This is a swelling of the spermatic veins. This is usually palpable above the testis as a bag of worms.

See a doctor: A physical examination is indicated. A list of possibilities can be given such as varicose veins (varicocele), epididymitis, hydrocele, inguinal (groin) hernia, etc. Needs a physical examination & sometimes testing to tell.