I'm 14, and I have very large veins aboove my left testicle but do not hurt. My left testicle also has relativley big veins behind it when they're not tight. I sometimes get achy feels (but not very often). My left testicle drops down alot further than my right one and like i said, it's very veiny, and i can't feel alot of my left testicle sometimes because of all the veins, but then sometimes i can, but the veins are still there. My left testicle is becoming alot smaller than my right one, and i can barely feel it because of all the veins etc like i mentioned above.
Sorry for bad explanation!
I'm really frightened to do anything with my gf because of this incase she sees, also afraid of what may be infertitlity - i don't know
As mentioned by fixmyface, veins in your scrotum are normal and on testicle usually hangs lower than the other, so nothing to worry about. If you are really worried your best bet is to have a chat with your doctor.