I have a strange thing that happens sometimes (not frequently).

It feels like a fish is flopping over in my chest. It starts with a tickle at the back of my throat, and thump, and it makes me cough. It comes and goes depending on some mysterious thing inside.

My doctor thinks it might be my hiatal hernia... which I have from a GI birth defect. My stomach is rotated 90 degrees to my back and doctor thinks the upper portion of the stomach is moving up into the chest and when I cough it goes back down.

Here is a diagram:
http://www.medicinenet.com/hiatal_hernia/article.htm

Mine always happens when I am lying down, too. Never sitting up.

I can go months without it acting up.

I think large meals, spicey food, alot of gassy foods (I can't drink carbonated beverages anymore), bending at the waist alot working (gardening/housework) may trigger mine.

It is a really strange feeling but I know it is not my heart.

Try taking your pulse when this happens. If it is normal, then that can quell your anxiety some.