Yellow vomit, no appetite, dog in heat: Is she sick or pregnant?

I am very sorry to hear that Perlita is experiencing these problems, I am sure it is very unsettling for you both. There are a number of potential causes of such clinical signs, including dietary indiscretion, pancreatitis, parasite infestation, infection, and ingestion of a toxin or poison. It is unlikely related to pregnancy if she is still I. Heat. However, some dogs can feel he unwell while in heat. I would believe the most likely cause is something she has eaten that does not agree with her.

Because you have only noticed these signs over the course of a few hours I would continue to monitor Perlita at home.

Rest Her gastrointestinal tract by holding back food for a few hours. Then you can continue with a bland and easily digested food of the boiled chicken breast and white rice diet and provide plenty of fresh water. I also find that boiled, mashed skinless sweet potato is helpful for diarrhoea if it begins.

If Perlita does not improve, has profuse vomiting or diarrhoea, you notice fresh blood, She seems lethargic, in pain or depressed I would advise a trip to the veterinarian.

Here are some links to helpful resources for you if she developed diarrhoea as well.

https://www.petcoach.co/article/most-common-causes-of-diarrhea-in-dogs/

https://www.petcoach.co/article/6-things-you-can-do-at-home-if-your-pet-has-diarrhea/

I hope that answer was helpful and more importantly that Perlita feels better soon. Warmest regards to you both.