Hi Ben,
I had a stress fracture of S.I.J in Feb last year. I went to see loads of specialists, chiropracters etc and only got diagnosed after beggin for a scan { which I had to pay for }. After a total cost of around £1000 and 3 months of messing around seeing different doctors the upshot was that they couldn't really do anything to help anyway. The initial crack in the bone had healed, but I was left with nerve and muscle damage, which was mostly sciatic type pain {nerve twitching etc}
The way I dealt with this was to wait until the chronic pain subsided then slowly start back. I also did an awful lot of stretches specifically for the lower back and strengthening excercises, to try and make the muscles of my lower back stronger. Some nerve and muscle pain still was present until August but I have none now and am back to running about 35 miles a week.
I am not saying that yours is a stress fracture, but that area takes a long time to settle down after an injury due to the amount of nerves around it.
Maybe just try to see someone because apparently sometimes the S.I.J becomes jammed and traps the sciatic nerve and a chiropracter/osteopath can unjam it which will quite quickly relieve the symptoms.
Good luck