About seven years after my J-pouch surgery I was discovered to have a "suspicious mass" on my right kidney. It was an accidental finding on a CT scan done for other reasons - it caused no symptoms. Based on how it looked on CT the right course of action was to remove it rather than biopsy it - the cancer that it looked like was completely curable at the stage it appeared to be at, and would be totally untreatable if allowed to spread.
The two key surgical choices were open vs. laparoscopic, and complete vs. partial nephrectomy (kidney removal). An open procedure is a difficult surgery to recover from, but adhesions can make the laparoscopic approach difficult or impossible (you can't tell in advance). A partial nephrectomy is technically more challenging for the surgeon, but offers the possibility of retaining function in that kidney.
I chose a laparoscopic partial nephrectomy. This combination is still pretty new, so only fairly young urologists tend to perform them. Mine ended up using a DaVinci robot, but that's really just the surgeon's choice of tools.
After all of that my mass turned out to be benign. I joked that it was "the crappiest great news I've ever gotten." I was completely cured of the cancer I never actually had.
I don't know if your mass is "suspicious" (it would probably require a CT scan to answer that question). If it is, and you've caught it in time, ignoring it could be lethal. I have no idea if your pain has any connection to the mass. For your sake I hope not.