I recently went to my physician about this exact problem. Not the most comfortable doctors visit I've ever had, but he said he was pretty sure it was a vericocele in my left testicle. This problem almost always happens to the left testicle as the left chord is longer and more likely to have complications. My MD also sent me to get an ultrasound directly after my appointment (another uncomfortable experience). Two days later i went in and had a consultation with my md and he said there was no cancer. Yay! From my experience, you can usually see or feel that there is an "enlarged" vein in your testi. Good Luck and go see your md!
I had a pain like that a few years back. Went to a urologist and he confirmed it was not cancer but a vericocele (pronounced veri-co-seal). Basically it is a vericose vein in your sack and can cause infertility. You can live with but the pain can be unbearable and radiate through the groin and back area.
I opted for surgery but there are non-invasive procedures. You can review them here: http://www.varicoceletexas.com/. However, I can't stress the importance of seeing you physician who will probably refer you to a urologist. Any competent urologist will send you for an immediate ultrasound to rule out cancer. Good luck and let us know how you make out.
First off, that was a very funny title. I started cracking up when I read "Ball Ache" on the forum...
I don't have balls (physically, athough I have been very bold before and told I have big balls...) but really pain in the testicle can be ANYTHING. It could be anything from trauma to testicular cancer. Go to the doctors. Read my other post about your cysts, I talked about how to get by the arousal awkwardness.