It's possible. Could be GERD, Gall Stones, I would be more inclined to think gall stones, cause if you lie down with GERD it generally becomes worse. It's good that the heart and all that checked out normal. I would highly suggest following up with a Gastroenterologist to explore GERD/reflux and even the gall bladder as well.
Here are some links you can read over as well, which may give you a clearer picture too.
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