A pulled muscle is a good possibility but...
Have you ever had a UTI or other kidney infection problems??? It sort of feels like you have been kicked in the small of the back... Nausea can also be present but not constant.... Can you be more specific as to symptoms???
I am interested in juice fasting. Do you mind if I email you some questions about how yours is going? Could I have your email addy? Mine is dietofworms1[at]hotmail[dot]com if you would be willing to answer a few questions.