For me, sciatica feels like a cramped muscle or nerve in one butt cheek and sometimes down the back of that thigh too. It FEELS like something I should be able to stretch out but can't. My SI joint pain is in the middle of my lower back (but just off to one side or the other of my spine). It gets SOOOOOO painful if I don't stay on top of it. I don't know if it is related to the SPD at all (I'm hoping THAT doesn't get as bad as last time, because I couldn't get out of bed or walk from one side of my house to the other without crying...it was horrid), but at times they definitely seem related (like when I keep on top of the SI pain, I don't feel as much pain or popping in my pelvis).
If it is your SI joint, you may find this relieving-
http://www.youtube.com/watch?feature=player_embedded&v=q_96kCy0kYY#at=115
...and stretching the hamstring on the problematic side (for me it switches sides, not sure what triggers either one) can help keep the joint from rotating.
My chiro has been helping me a bit this time- last time chiro care didn't do a dang thing for me- so I'm hoping that just staying on top of it overall will really make a difference for me this time. Oh, and maybe having a girl will make a difference...IF this one is a little "she"...because my girls were significantly smaller than my son, but I carry very low each time, and I think that contributes hugely. I also found an SI belt to provide some form of relief but I couldn't wear it all the time.
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