My dog is in heat, lethargic, and not eating. Should I be concerned?

Dogs have heat cycles and not menstrual cycles, however it is possible she is going into heat and hence the behavioral change, though she shouldn't refuse to eat, or be extremely lethargic during. If it persists for more than a day, doesn't improve, or her symptoms worsen, it is best to seek out veterinary care to rule out issues such as an infection or another illness that could be causing these symptoms as well.

It's possible, if she is in heat and was outside, that a male dog came into contact with her. If she is now acting very ill or lethargic, it would be best to seek out veterinary care to make sure there isn't another underlying cause, injury, or illness causing her symptoms.

Drinking more than usual, loss of appetite, or vomiting are all classic signs of an infection if the uterus also known as a pyometra. These are more common in older dogs. These can become very serious and even life threatening if left untreated. I would strongly advise that you gave a veterinarian examine Misty right away.

I would be concerned that Bell hasn't returned to normal after 12-24 hours. While heat injury is possible usually dogs have issues sooner than now such as seizures, diarrhea, bruising and/or collapse. She could have a problem with her GI tract, something stuck in her esophagus or even a respiratory issues causing her changes. If she continues to cough, seems lethargic/lack of energy, or has any vomiting, diarrhea or continues to have a decreased appetite today she needs to be seen by a veterinarian. I would try calling your veterinarian today as they sometimes have an oncall service or an emergency veterinarian on call to answer questions as well.

Dogs can feel lethargic and not want to eat during their heat cycle especially their first one. However, this shouldn't last long. Vomiting is a bit unusual for a heat cycle. She should get a physical examination by a veterinarian to make sure she doesn't have gastroenteritis or pancreatitis.

From the sign you are describing it is quite difficult to determine the underlying problem. General signs of lethargy could be commonly due to high temperature which may be related to an infection. In puppies gastrointestinal and urinary tract infections are by far the most common type of infections. If the signs do not resolve in the next 24hours or if Luna is showing any inappetence, more evident lethargy or any other signs (vomiting and diarrhoea for example) I would strongly advice a veterinary check up.

I'm very sorry that Summer is not acting herself. Unfortunately, her symptoms are very vague and don't point to any one illness or disease process. Pretty much anything can cause lethargy and not eating. I strongly recommend you have her examined by your vet to help figure out what is going on with her especially if it has been a few days already.