Re: mystery problem
I would assume, since nothing shows up on MRI or X-ray, that your girlfriend has pulled a muscle or something similar. This can take a long time to heal. If she has rested the injury for an adequate period of time, and it still is sore and with rest, is not improving, I would suggest you see an orthopedic doctor who specializes in Sports Medicine. They are about the only medical specialty that pays much heed to muscles and soft tissue.
I can't speak specifically to your girlfriend's issue, but I know with back issues in general, particularly those who are injured during exercise, people tend to think they are "resting" for an adequate period of time. But in reality, they rest until it starts to feel better, but they are so eager to get back to the gym or the bike or whatever, that they start right back into the activity too soon, and, with almost the same intensity...and the injury begins to hurt again.
If someone is unable to do an activity for more than about 2 weeks, he/she needs to take the attitude that he is starting all over, and resume the activity slowly, and gradually add minutes over a longer period of time.
The body deconditions much faster than the time it took to reach that level of initial conditioning.