The source of yesterday's problem.

After speaking to Tim Edward, a teacher at school and also a very good fell runner, I have managed to find the cause of yesterday's torture. After finding out about my lower back problems, he immediatley said, 'Have you been doing any core conditioning stuff?' I immediatley replied 'yes', as I have been doing this strength and core work over the past couple of weeks. 'Ah, it will be that causing spasms in the bottom of your back; Too many situps and the like leave your core too strong for your back muscles. Either cut down the number you're doing and build up slowly or cut the core stuff out completley. I had the same problems at Kilnsey a couple of years ago.'
So there's the answer! Having not suffered with this problem before, Tim's suggestion makes perfect sense. I'm glad I know what is responsible anyway and I will certainly address this by cutting down on the core stuff. He also mentioned some excercises which will work both the core and the back muscles, which in turn wouldn't lead to muscle inbalances. He is going to dig a sheet out for me which helped when he had the same problem. Until then, no more core stuff for me!