Its probably varicocele, I have it too. Never bothered to get it looked at, but its been like that since my pre-pubescent days and hasnt really changed, so I don't think its a big deal. I have been checking for changes / legit lumps on my nuts, but its just backed up veins that make it feel like that.
Good decision getting it checked out though. Surgery isn't necessary for most cases, but yours sounds worse than mine, my veins return to normal when I lie down or relax. Maybe I should have the doc look at it the next time I'm at the doctor.