I also had that once Gavin. Definitely not dehydration. I was worried I was pissing blood or something it hrt that much.
I had put a new Brooks B17N on, but didn't bother checking the new height. Since the rail to arse interface was noticeably bigger on the Brooks than the old saddle, the saddle height was effectively increased.
The combo of a too high and new Brooks was agony for about an hour afterwards, as you say, like peeing razor blades. I thought I had done myself a serious injury.
I realised my mistake and dropped the saddle height a spot and it was fine thereafter (even before the Brooks had 'worn in').
Might be worth a try Gavin?