Two steps forward, one step back

Well the bad news is that Kaydee has slipped, and in so doing hurt her back again. Luckily for us Stuart McGregor was available to help out. He said her symptoms are typical of a slip where the front legs go one way and the back ones go another.
This seems a good point to refer back to something Stuart said in his seminar that I found particularly interesting, so I have done a little more googling to find out more.
He talked about why it hurts when a joint becomes immobilised. This can happen if a joint is hyper-extended (taken beyond its natural position), because muscles around the joint then 'clamp down' to protect it from damage (the feeling of muscles going into spasm). The theory for this pain is explained by the The "gate control theory" proposed by Melzach and Wall (1965).
In the gate model of pain, the neural fibers that carry the signal for pain and those that carry the signal for proprioception* are mediated through the same central junction. Because signal transmission along pain fibers is slower than transmission along proprioception fibers, the gate model suggests that intense stimulation of proprioception fibers can block the slower-moving pain signals.

This means that when your vertebrae or other joints are free to move normally, there is proprioceptive feedback constantly occupying your neural pathways, so you feel no pain. However, if a joint becomes immobilised there is no proprioceptive feedback mechanism occurring to block the pain signals, so you feel pain.

The "gate control theory" proposed by Melzach and Wall (1965, 1979) identify cellular structures in the substantia gelatinosa in the gray matter of the dorsal horns of the spinal cord, which act as a "gate" that when "open" allows the perception of pain through the reticular formation and thalmus, with response from the limbic system, among others. When the mechanism is stimulated to "close," the gate inhibits the perception of pain. Two types of fibers as identified by Melzach and Wall affect the way pain "gets past, or affects the gate." Small fibers affect the "T cell" and act to "open the gate" when noxious stimulation occur. Larger fibers affect the "SG cells" located in the substantia gelatinosa in the gray matter of the dorsal horn to act in response to proprioception and touch during changes in body movement. They inhibit the T cell response and "close the gate." This in theory changes the course of noxious or painful stimuli, and causes a natural analgesic effect. "Inhibition of pain is increased by stimulating the mechanoreceptors (larger-fibers) in two ways: by movement and activity of muscles and joints, and by manipulation of the joint."
Melzach R, Wall PD. Pain mechanisms: A new theory. Science 150:1965; p. 971.
Melzach, Wall. Gate Control Theory. Churchill Livingston; 1979; pgs. 21-23.

What this means is that by correct manipulation to re-establish normal joint position and movement in a joint, the pain disappears.

*Proprioception
Proprioception refers to sensory information telling us about our own movement or body position, so that we do not have to look to see where our body parts are to know where they are and what they are doing. It provides perception that helps integrate touch and movement sensations. For example, it allows us to walk in complete darkness or touch-type. Receptors for the proprioceptive sense are in the muscles, joints, ligaments, tendons, and connective tissue. The stimuli for these receptors are movement and gravity. Proprioception, the "position sense," sends messages about whether the muscles are stretching or contracting, and how the joints are bending and straightening. Even when we are motionless, gravity stimulates the receptors to create proprioceptive messages without our being consciously aware of them.

Kinesthesia
Kinesthesia is a term that is often used interchangeably with proprioception, although use of the term "kinesthesia" can place a greater emphasis on motion, and they seem to be separate physiologically. Proprioception and kinesthesia are seen as interrelated and there is considerable disagreement regarding the definition of these terms. Some differentiate the kinesthetic sense from proprioception by excluding the sense of equilibrium or balance from kinesthesia. An inner ear infection, for example, might degrade the sense of balance. This would degrade the proprioceptive sense, but not the kinesthetic sense. The affected individual would be able to walk, but only by using the sense of sight to maintain balance; the person would be unable to walk with eyes closed. Proprioception is, in essence, a feedback mechanism; that is, the body moves (or is moved) and then the information about this is returned to the brain, whereby subsequent adjustments could be made. Kinesthesia is a key component in muscle memory and hand-eye (or paw-eye!) coordination, and training can improve this sense. For example, the ability to catch a ball requires a finely-tuned sense of the position of the joints. This sense needs to become automatic through training to enable a person to concentrate on other aspects of performance, such as being aware of where other people and objects are.