Do you have steel door? You could put them in the door jamb and SLAM! Problem solved.
I saw some traps the other day that made my Shenlong.
I get the same thing, sometimes, it's the because ejaculation tends to lend to your ball tightening your ballsack in order to prevent any additional semen from leaving so that it may regenerate. In your case I believe you have jacked of so much that the vain which semen flows from your balls is sore from continuously being tighten causing the pain you feel. Let your balls rest, sushi.