I found a lump on the back side of my left testicle, near the bottom. Is this the epididymis?
No, the epididymis is located on the back side of each testicle, but near the top. It feels soft or spongy, not hard. Since you found this lump on only one testicle and in an odd place, you should see a physician to have it checked out. The primary concern is testicular cancer which effects young men. It is a fast spreading cancer, so you need to have it checked as soon as you can. I pray that it turns out to be something harmless.