I agree with @Afraser re possible intake of too much water.
Maybe also consider adding a watered down 'exercise drink' to your drink regime? When drinking copious quantities of fresh water, it also flushes out a lot of the necessary 'micro' nutrients that are essential for the healthy use of the body ..... a friend ended up with a pace maker after drinking too much 'clean water' (after doing a heap of dragon boat racing!) She actually needed the electrolytes to be replaced, but had watered them down instead! :(
Take care xx