By what mechanism does prostatitis cause lower back pain?   It seems
(to my lay-knowledge of anatomy) that the prostate is the wrong side
of the lower colon to cause back pain through physical pressure alone.
So I imagine there is some sort of biochemical interaction.  And yet
the back pain I experience is indistinguishable from back pain I had
when much younger from playing tenis, lifting things etc. Can someone
explain please?

As background, I am 46. I was treated for prostatitis a year or so ago
after being unwell for some time. Since then I have returned to
swimming and some running, but I find that if I do too much (not
really much compared to before becoming ill) my prostate swells, I get
lower back pain and anus pain.  Fortunately, I have very little
trouble passing water.  The latter seems atypical from what I have
seen reported on the internet.