If you're certain it's only since the vasectomy, and I'm sure you are, that's unusual and is a new one on me. I have heard of undescended testicles in young boys and even when your balls will temporarily "jump" back up in you sometimes. Mine go back up inside sometimes when masturbating.
What you've described since your vasectomy sounds unusual but doesn't sound harmful. A doctor can tell you why I'm sure.