Okay, so it's been about 4 years since my vasectomy and I also started having signs of a lump that has since grown to the size of a 3rd nut...bonus. I got an ultrasound and was told it was a cyst. The more times I search on it, the more I seem to find similar circumstances with no Dr. corroborating that this is common. Nice to know it's not just me. I warn friends now :)
I am sorry I don't personally have an answer for you. I tried posting a question on here about my post vasectomy pain and no one responded to it. It seems like there isn't much information on here for guys who got a vasectomy.
I suggest you google "post vasectomy forum" and see if you can get a forum where someone can give you some information, like I did : )