Help with Zazen practice.

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For some time now, I've been getting a muscular pain about 15-20cm below my left shoulder blade, just left of the spine, and I wonder if I'm sitting off centre. At first I thought it was just a one-off or occasional occurrence, but it has been going on too long and too frequently for that. I push the buttocks back and the belly forward every time I sit to ensure I'm sitting with a straight back and rock from side to side to centre myself and get the "funnel" effect for the torso, but this pain keeps recurring. I continue zazen in spite of this, but naturally it would be better physically, mentally, and spiritually if it did not occur at all. Has anyone got any ideas on this? I hope I've opened this thread in the right section.