What were you taking the antibiotic for? Did you get a diagnosis?
It sounds like your doctor did a good job ruling things out. And all of the serious things are covered to NOT be the cause. One of the causes of pain in one testicle is basically injury. Do you think you could have just been a little rough with yourself? There is also something called epidymitis which is swelling and irritation. While it can be caused by things that have been ruled out such as blockage or infection, trauma can cause it too. Ice it, rest it, use iboprofen if you can take that, and it should resolve if your doctor agrees that is the issue.