I second Kitt's welcome :)
I am on Nexium 40mg per day, and I sometimes get the stomach 'tightness' you described. It usually happens for me in the morning AFTER I have taken Gaviscon the night before. I suspect it has something to do with me being nauseated, because it feels like the feeling I get after I've thrown up recently; it usually calms down if I don't move, but if I move, it starts again and lasts for a few seconds and up to about a minute.
I wish I had a better answer for you! You might also be describing bloating, which I get from drinking too much water.