What Causes The Swelling Of The Right Testicle During Intercourse After Vasectomy?

Hello, my husband is 34, 6 ft 5 and 300lbs. He had a vasectomy 4 years ago. When he had the vasectomy his right side did not take the doc said and he had to have that side done again. Now four years later his right side will swell up. When he ejaculates it goes down. This only happens before intercourse or every 5 days or so. This seems odd to me and I was wondering if there is a possibility that side has come undone again??? thanks