What Could Cause Mild, Off-and-on Discomfort In My Left Testicle With No Noticeable Lumps Or Swelling?

I’m having discomfort in my left testicle. It’s been going on for a couple days, but is mostly off and on and I only really notice it when I think about it. I can’t feel anything out of place, but the pain is feels like a mild version of the pain when hit in the testicles