Sunshine - it doesn't sound like anything muscular. Perhaps a trip to the Dr might be more worthwhile than a trip to the osteo.

Gall stones, kidney stones, pancreatitis...they are incredibly painful and the pain in high in the chest and of course radiates to the back. You might have a stone stuck in your CBD.

Being pregnant increases estrogen and flares gall stones. Maybe being pg with DD2 caused a flare up. Perhaps you had smaller stones and one got stuck on the way out, but eventually passed through. They are not a sign of poor diet or unhealthy living.