That's good that it isn't pyo, but not good that it is endo.
I know endo well as I have it!
This is a potentially very painful condition where the uterine tissue begins to grow all around the internal organs -- typically it attaches to the stomach, diaphragm, bladder, bowel, intestines... it can even penetrate thru the diaphragm and enter the lungs where it can cause a collapsed lung (my father, a respiratory specialist, was actually the first person to describe and name the lung aspect of this very strange condition in the 1970s).
Every time Kayla goes into a heat the uterine tissue will behave just like the uterine lining and swell in preparation for the possibility of puppies, which can be very painful in itself and is almost certainly why she has been off colour. Then, when the lining is shed all the tissue within the abdomen ALSO bleeds which causes cramping and discomfort at best and more serious problems requiring surgery, at worst.
In humans the effects are most noticeable during menstrual cycles and can require hospitalisation. The abdomen is very unhappy when there are fluids like blood seeping inside and the reaction is the body tries to fight off what it thinks is a foreign body. Infections are possible.
The human treatment is either full hysterectomy or the surgical removal of the endometrial tissue, which can be a major or minor operation depending on how much tissue and how many adhesions there are (where the tissue is attacjed to internal organs). Hysterectomy (eg spaying in a dog) removes the cause of the endometriosis -- eg the heat cycle.
Endometriosis tends to increase in severity over time as those of us with it can unfortunately testify. If she is already showing signs of it now you would probably want to act to remove the cause (eg have her spayed as soon as you can so she doesn;t go into another cycle and go through all of this again).
Most of us with endo find doctors do not know very much about the condition and even some specialists are unaware of it and what the symptoms are. Endo often acccompanies ovarian cysts in humans; I don't know about dogs. These are sacs that form around the ovary and fill with fluid and can become very large. If they burst, they can be life-threatening (like appendicitis), so doctors tend to want to remove the cysts when they occur. Again this requires abdominal surgery.