Hi evosy1978

Without doing and examination I wouldn't really be able to tell you the offending body structure.

Yes it could be one of the muscles around the pelvis or perhaps something to do with the Wikipedia reference-linksacroiliac joint. However low back pain frequently refers pain to that area and as you have been having such pain on and off I think it would be quite likely to be related to your low back. That is simply based on the rule that if you can find one cause for all the phenomena that is more likely than multiple causes.