Pain from 'dem stones moving around in the bladder I think - but I don't really know, since the USG after passing didn't indicate anything.
For a long time any significant g-forces would cause pain in the abdomen, which I unwittingly started ignoring at one point. But the fear of that pain stayed and I couldn't identify where it came from.
When I the plane started its ascent and I felt nothing, I connected the dots - especially that the passing and flying were just a few months apart.