Gave a urine sample which was disgustingly conclusive (as in, pee shouldn't be that colour). Also had a high temperature and fast pulse rate. Based on the symptoms and some prodding to find out where it hurt, the doctor declared it to be an infection in my left kidney (pyelonephritis - cystitis that "heads north"). I was given a massive dose of IV antibiotics, a blood test and an X-ray. Amazingly, the blood test was almost normal - white cell count and albumin a bit high, everything else within the range of normal. Also the X-ray did not show any kidney stones. As the IV antibiotics had helped the pain a lot, I was allowed to go home with lots of antibiotics and strict instructions to come back if I got worse or didn't get better.
It seems I was lucky - the Wikipedia page suggests that a lot of people with pyelonephritis have to be admitted to hospital for a couple of days. Apparently I went at just the right time before the infection had really taken hold. The scary thing is that I must have had cystitis for a few days (or weeks?) and not even noticed. Yeah, my pee's been a bit cloudy and smelly lately, but then my gall bladder was acting up. And yes, I've been in pain - but I had a period, and before that I just had being in my body - all the aches and pains associated with dragging myself halfway across London three days a week. I'm wondering if this means that, rather than having a low pain threshold like I've often thought, it's actually high. (Not noticing cystitis?)