My dog keeps vomiting?

Well, my dog usually lives up at my grandparent's house because their dog died right before we got our dog so we came up with the idea of her living up there most of the time. Sometimes like once every two weeks she comes up to our house to be here for the weekend, but since its summer, she comes up like once a week. She's never vomited before here, but we had her yesterday, and she threw up once outside, and we don't give her anything we eat, just dog food, which is pedigree. My dog's favorite person is my mom and she's at work all day, but so is my dad and she likes us kids anyways because we're the only ones there so that wouldn't really be the problem since she's been up here when they were both at work before, so yeah. Then I woke up today and went outside of my room and stepped in vomit! Ew. Then my sister said there was vomit in her room, too. So that must have happened overnight since i remember her coming into my room at night and when i woke up she was there. She's only eight years old and the vomit was yellow outside, outside of my room was greenish-clearish and the vomit in my sisters room was brown, just in case anybody needed to know. Also, she is acting really sad because her real personality is crazy and still eats but not too much. I know if she ate too quickly and then went outside to play her stomach would flip over or something so we wait about an hour before playing after eating.Sooo i love my dog so helppp?!?! Oh yeah sorry its so long!