It means that he doesn't know, but thinks it's a mecanically based problem. It almost certainly is a musculoskeletal (mechanical) problem, in that he's most likely right, but question is what sort? And it may be too early to tell.
The 'crack' suggests that you either have a 'locking' of some of the small facet joints at the back of the spine or it could be a disk injury and these can take several days to develop enough to be able to be identified. [sm=scratchchin.gif]
As you probably know diazepam is a sleeping tablet, so at least you can get some sleep, so may save your sanity. The down side is that the sounder you sleep, without moving, then the greater the chance that the inflammation will build up in the area, so could be worse in the morning. [
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Keep moving as best you can, without overdoing it during waking hours and as several members have said book yourself in with an osteopath for next week. Although they may be able to do something this early, it's usually better to leave a few days to settle a bit.
I tend to use some western acupuncture pre treatment for really acute cases to calm things down a bit and get some pain relief, before doing much else. So if it's really painful it may be worth looking for an osteopath near you that does both. Massage & other soft tissue work may help calm it down a bit also, and you could try hot &/cold on it, although if it's a deeper injury such as a 'slipped' disk, then it may not have much affect.
Good luck - Andrew.