Small issue with Sonny?

I was just wondering what might be causing Sonny to vomite. It's never after eating either. He's now been doing it for the last 3 days, 1 time the first day, twice yesterday, and once this morning. It's usually a very mucusy, frothy yellow liquid, not solid. And when I mean frothy, it's real bubbly, like when you pour rootbeer. But it's not like carbonation where it continues to bubble, it's just how it looks when it comes up but in a bright yellow color. Any ideas what this could be from? He's eating fine, peeing and pooping fine, and doesn't appear sick and is full of energy. It's just weird. I was getting ready to get up this morning and he did it just before I got up out of bed. I also decided to let him sleep with use last night instead of his crate, was taking a big chance as he might pee or poo inside at night, but he didn't. He slept on the bed most of the night with the girls. As for now, he's outside with the girls, wrestling with Chloe, lol. I will see if I can get a hold of the vet today and ask them as well, but was just wondering if anyone here might have any ideas. It's like all of a sudden it just started.