Thanks to everyone most nicely for the answers, but I think that maybe the main problem for me is - really, I don't drink much fluid.
On Saturday I'm heading to Stol, so I'll try to drink more. Otherwise my light snack is most often also tuna and a slice of rye bread, plus bananas and dried fruit, because I too can't stand the so-called heavy stomach. Still recommending myself for some suggestion.
Thanks everyone, Andreja.