Ok, hear me out.
A lot of times when I am having heartburn, I'll make a movement and then feel the heartburn more accentuated. I'll then repeatedly do the movement to check, and sure enough I'll recreate the heartburn feeling each time.
As an example, I just took a shower and at one point lifted my left arm down and back or something. I noticed it gave me the heartburn pain, and so I did it a few times in the next few seconds to 'check' and sure enough it gave me heartburn each time I did it.
It makes me think it would be muscular, but this only happens when I am already having a bit of on and off heartburn (for 10 minutes or a few hours or what-not). If I am having a period with no heartburn, I'll never get the accentuated feeling at all. In other words, I must already be having a bit of on and off heartburn for this to work.
Its happened enough times for me to finally want to ask others if this has happened to them to. I don't know why it would work this way, but it is definitely something I have noticed.