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Argos did the exact same thing when he was a pup (he had the problem when he was younger than your girl). He still does it from time to time and I agree that it is common and often nothing to worry about. BUT, in Argos' case it definitely was something to worry about. Fortunately, I was going to an extrememly careful vet at the time and she was determined to get at the root of the problem. She tried a number of things that provided no results. Finally, we x-rayed his tummy. Argos had swallowed a metal bottle cap and those sharp edges were scraping his insides. It had to be removed endoscopically (a vet stuck something down Argos' throat and pulled it out). If we hadn't x-rayed Argos and removed the bottle cap he would have kept on hurting and serious damage may have been done.

It happens in very early morning and it consists of foamy yellow fluid, no food solids at all.

What indicated to me that this was a problem was that he was throwing up earlier and earlier every day. Towards the end he was throwing up before 6 AM. Dogs often throw up on an empty stomach, in fact Argos still does. But, throwing up that early is unusual.

She did get a little piece of a plastic ketchup bottle cap, the part that flips open, and swallowed it before we could get it, this happening about 5 days ago.

I was told that plastic does not show up on x-rays (those of you who know what you're talking about please correct me if I'm wrong). If that's true, the bottle cap won't show up unless you use a special procedure where the vet basically x-rays dye as it moves through the digestive system (I forgot the name of the procedure).

I will probably take her to the vets, but wanted to get some opinions anyway.

The vet is a good idea. Better safe than sorry. Trust your instincts. If this vomiting is a new thing that occurred suddenly I would be especially cautious.