I also experienced lower back pain and read a post a while back that explained what it was. It is obviously your body detoxing....your kidneys, I think? Whatever organ that is back there is detoxing and it causes discomfort. It definitely made me feel better to know that even though it hurt, it was for a good cause!
The grouchiness is actually my only complaint about this detox. I havent been able to find a solution for it. There is something about food that just gives you those happy "endorphins" that you dont get with lemon juice and maple syrup. I have to imagine myself happy and try to act upon it whether I feel like it or not!