What Causes Swelling In Testicles Post Masturbation?

I have a swollen right testicle but it seems that it only swells when I masterbate. I don t always ejaclelate, I wait a little while or wait days at a time. But after I do this and after I ejaclelate it seems that my right testlce will swell and then I can feel a lump on the top part of the testlce. What do you think this is?