Thank you very much for the useful information David.
This is most likely a result of infection involving the testes and its associated structures, epidymitis, orchitis, or epididymo-orchitis.
You should have rest.
Drink lot of water.
Avoid sexual activity for few days (week or so).
And use Motrin or Aleve to control any ongoing inflammation and swelling.
If despite these measures, the problem persist, does not improve or get worse, see your doctor for a clinical evaluation in terms of an examination aided by tests like ultrasound of the testes (to make sure this is not the circulation issue to the testes).
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