SI is sacro iliac joint. SI dysfunction can mimic sciatica.
I have 2 herniated disc at L5-S1 and L4 At first 2 doctors xrayed me told me it was my sciatica nerve and gave me meds which I couldnt take at work. (pain pills) They gave me steroid shots, antiinflamitory shots and pain shot none helped. Even ER dr. said same thing thanks to another Dr. doing a MRI which showed herniation and other problems which my systems were that of what you are saying. I will be having surgery in the next week. Let me know if I can help.
So, what did the doc say? Sounds a bit like sciatica to me. Interested to hear the diagnosis as once or twice this has happened to me and I have assumed it was from sitting too long at the computer!