I am here, I am licensed in Michigan & Ontario, Canada.
I am very sorry to hear about Lucy's lethargy, loss of appetite, increased thirst, and loose stools with blood and mucous (gelatinous component). I understand that she has a rash, but I am not sure that is connected, unless she is in liver failure or has diabetes which predisposes to secondary infections.
Of course anyone would be concerned about blood in the stool. The fresher the blood appears the closer the source is to her rectum.
The important factor in all of this is how your girl feels otherwise. Given her loss of appetite and lethargy along with a ravenous thirst I am very concerned. I suspect her extreme thirst is related to dehydration from profuse bloody diarrhea, but it may also be indicative of underlying ketoacidotic diabetes, or organ failure.
There is a condition in dogs called hemorrhagic gastroenteritis. These dogs have profuse dark, jelly-clot like, bloody diarrhea. They are very sick dogs and get dehydrated quickly so they need immediate care. This sounds very much like what is going on with Lucy so an emergency veterinary visit is best today.
Colitis or inflammation of the large intestine, can also be due to stress, a change in diet, parasites, a viral or bacterial infection, inflammatory bowel disease or even a food allergy. Rarely a mass in the colon can be the cause, or exposure to anticoagulant rodenticides. But these conditions don't make most dogs as sick as she is.
I highly recommend an emergency clinic visit.
If that absolutely isn't possible let me know, and I can give some recommendations, but I am worried they will not be enough to help her today.
Best of luck with your pup, please let me know if you have any further questions.