This Doesn't Happen Every Night, But Sometimes When I Try

This doesn't happen every night, but sometimes when I try to fall asleep I get an intense discomfort in my stomach. It is similar to nausea but not quite that. Changing positions makes it go away for a few seconds, but it then comes back. It goes away completely when I sit up or stand. It also only seems to happen when I am lying with my eyes closed, if I am lying down and doing something else (on my phone or reading a book) then it doesn't happen. Because of this I'm thinking it might be psychological but I'm just not sure!