I’m a PCa patient myself, had radical prostatectomy 2 yrs ago. As long as PSA is undetectable, you should be OK. In virtually 100% of cases, presence of residual cancer would cause a PSA rise. Also, I doubt any small am’t of cancer in that area would cause pain.
As for what’s causing the pain, I can tell you this: Prostate Cancer surgery is a pretty complex & delicate procedure, a lot of tissue is removed along w/ the gland itself, and there can be lingering effects for years. In my case, my urination feels a lot different than before, it doesn’t really hurt but often feels almost as if I have a bladder infection. So I think you’re prbly just experiencing lingering effects of some nerve damage from the procedure. Hopefully it will become less noticeable over time.
I guess you could always ask your Urologist if there’s any type of scan that could reveal the source of the pain, but I have a feeling it’s not likely to show anything...