Most common causes for testicle pain is: Injury, Infection or inflammation, Testicular torsion. If there has been no injury or any possible reaction to anything or any type of bug bite you should be seen soon. With you saying it is 2x the normal size, I would either recommend a call into your doctor (since he knows your medical history) or a trip to the ER.
It could be just fluid or it could be something more serious.
Here is also a link you can read a bit more on testicle swelling as well.
Read hereHope this helps, If you need anything else please feel free to ask. If you found this helpful don’t forget to click the *accept* button
