Hey Caribou. As a guy i know how annoying random testicle pain can be! I've had on and off bouts with it myself over the past few years and its given me real stares at times!
First things first, testicular cancer rarely actually causes pain in and around the testicles which kinda helped me calm down a little bit. I've been to the doctors twice myself and also had one Ultrasound over a year ago, which came back perfectly fine.
When you get anxious about your testicles, do you self check them a lot? When i have periods of worrying about testicular issues, i tend to check them A LOT and that in itself causes them to ache, so that could be the issue regarding the pain you have.
Finally, if the pain you are experiencing is more so in the 'sack' more than the testicle itself, this is comforting news. If this pain continues i would definitely go back once more and request an ultrasound screening which will put your mind at ease.