What Causes Pain Lump On Testicle?

Hi, on Saturday morning I slowly started getting a pain in my right testicle, I don t usually examine my testicles on a regular basis so I don t know if there was anything there before this. I felt around and found a kind of soft but firm lump at the top of my ball. It s very achy painful to touch but most of the day it goes without notice. I was just wondering if you have any clue what this is. A few years ago I had a problem I think with the same ball where one morning I woke up and it was so painful and just like inflated to about 2 or 3 times the regular size. I didn t go to the doctor and within about a week to a week and a half it had gone down to normal. I ve had no problems since. I m an 18 yo healthy ish guy, definitely don t have an STI but have regular sex with my girlfriend.