After some thought, the answer is that the most likely reason behind the swelling is structural - the discomfort of the spinal erosion plus the missing disc causes an altered gait that affects blood flow.
Pressure is being placed on the circulation to the hips.
This is not fibromyalgia, although it is possible to develop fibromyalgia (which is a lack of oxygen to areas of muscle) in such situations.
Some work with a cranial osteopath, or at very least a physiotherapist, is necessary.
Ultimately, though, only physical therapies can help structural problems.