Morning Sickness???

Hi all,
A few times a month, first thing in the morning...around 5am...my dog is sick. All it is is yellow phlem. Could this be from an empty stomach all night? She gets dinner around 5pm and a couple milk bones before bed.
I called the vet for maybe a general idea of what it might be, all she says is bring her in for a gazillion tests and we''ll see. Right.....I wish vets were more like family doctors in giving advice....I don't mind taking her to the vet, but I can't afford a thousand dollars worth of tests just to find out she has a sensitive tummy, and maybe it's a type of breed thing. She's a perfectly normal, healthy, active dog in every other way.
Oh...she's a Cocker Spaniel and will be 2 years old on the Fourth of July.
Any ideas would be greatly appreciated.
Than you all in advance,
Lee Anne