I have the same symptoms.
It started when I noticed by looking in a window I was running past that I might not tilt my pelvis enough and subsequently started to concentrate on that. Perhaps I am overdoing it ??
I actually regularly stop at lightposts and lean my back against it to make sure my pelvis is in allignment with my spine. I find this pelvic alignment focus one of the most difficult aspects of chi running. I usually make the allignment statically as described in the method (book & DVD) but as soon as things get dynamic and I have ran a while I feel it went out of the window without me noticing. So right now I am perhaps ovedoing it. It also feels a bit unnatural, isn’t the curve in the lower spine a normal anatomic feature? I would easily agree with the pelvic tilting if I felt a real improvement in mobility ease and relaxation, but I don’t, on the contrary.
I am quite puzzled, what’s the expert’s take on this fundamental one ?