I don't understand the lower back pain............ that makes absolutely no sense to me. Unless of course it comes from not sleeping and tossing and turning all night trying to get to sleep *shrugs*
The "fire" sensation can be explained by the fact that this acts as a thermogenic, which by definition will increase your body temp. Hard time sleeping, your probably just taking it too late in the evening. Keep in mind the C stands for caffeine. I know I sure have a hard time sleeping when I've just popped 200mg of caffeine into me, but that's just me. I also found I got a headache, but that was only when I was dehydrated. If your taking an ECA you need to make sure your drinking more water than normal, and also make sure your not taking more than the recommended dose because this stuff does have the potential to be dangerous. If it's unusually hot outside, drink even more water.