I used to get this as a child (and sometimes as an adult as well)and my DS, also 6, complains of tummy aches when he's worried or stressed. I told him very warm baths and/or hot water bottles used to help my tummy relax, and now this really works for him! So if he's worried about school the next day, I pop him in for a warm soak and then talk about what's worrying him. I'm sure any relaxation ritual will help your DD - a bit of 'yoga', or breathing, or weak tea and a cuddle... strategies she can keep using as she gets older. But tackling the root problem is always the most helpful thing, of course.