Do you do any energy work for clearing blockages - are you blocked at those places? I may have to lookup in a book whether these points are "pathway terminators".
I do not know but I have a similar phenomenon, at least to your last description. It originates at my lower back, near the mingmen - the opposite point of the navel point of the lower tantien, where it joins the spine, and near the kidney gates. You could call it spontaneous ventings. I had those spontaneously sometimes. It felt like a shudder, first venting in my back, and later on "farther" when I started cleaning my energy pathways. I do not trigger them, though. According to a book I read these spontaneous ventings can happen if excess Chi from the kidneys (prenatal essence Chi) is not stored in the lower tantien. This phenomenon does as well not go "the full way", but its "range" has increased with a more unblocked energy body.
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