Occasionally i get a burning spot in my lower back (si joint?) and at the same time a burning spot on my outer shin on the same side. any ideas?

Sciatica: This could be a herniated lumbar disc or degenerative spine in the lower lumbar spine irritating the nerves called a radiculopathy. See an orthopedic spine surgeon, neurosurgeon, or a neurologist to get a diagnosis.

L5-S1 Radiculopathy : The symptoms in the si joint are likely due to referred pain from disc disease (usually l5-s1) or spinal stenosis, or osteoarthritis of the hip. Its less likely to be coming from the si joint itself since the symptoms are felt in the shin. A history & physical exam will likely reveal the answer. X-ray is recommended. Mri and a nerve study will help confirm the diagnosis.