Weird
If it is just liquid though, are you sure her stomach hasn't been empty?
If it keeps happening and you can't figure out why, I'd say it is vet time (sorry I know you were just there).
The only other thing I can think of which I know doesn't apply in Layla's case is, has she been licking at something that she isn't supposed to? Or maybe an ingredient in her food in bothering her or her vitamins? It would probably be more than liquid if that was the case though.