Diagnosis opinion (pneumonia or strep zoo?)

Yesterday at approximately 11pm I brought my 4 year old male German shepherd to the emergency vet. The prior night he did not eat dinner, which every once in a while he skipped a meal and was fine. Yesterday he seemed depressed and slightly lethargic and refused to eat even his favorite treats. Due to this I was going to call today (monday) and set up an appointment with his vet, however about 1-1.5hrs before I took him to the emergency vet he became very lethargic and had labored breathing. He also seemed to have some weakness in his hind end but could still walk on level ground. Once we got to the emergency vet they took us in immediately and did an exam to which they found he had pneumonia. We talked about at home care vs staying overnight and I opted to keep him in their care so he could receive around the clock care and IV fluids due to his lack of appetite. They seemed very confident that he would recover enough to come home at 7pm the next day, but about 3hrs after I left for the night to hopefully get some rest I got the dreaded phone call. His condition slightly improved at first (respiratory rate was 120 at intake and was at 60 after a couple hrs of care, temp on arrival was 104.4°f and also returned to a normal range) but his breathing became much more labored and he started to have blood come out of his nose and mouth with extensive drooling. I rushed over to be with him and they did one xray which confirmed fluid was isolated to the lungs and not the chest cavity (they were hoping to be able to drain some if it was). Due to the nature of his condition and being after hours they did not have a radiologist to review the xray as well but the vet also believes his heart was slightly enlarged. His blood had stopped clotting and he developed an irregular heart beat. Due to these issues and the extremely poor prognosis I was told we agreed euthanasia was in his best interest due to the amount he was suffering and slim chance of recovery. My question is if you believe this is a viral pneumonia or possibly Strep Zoo? Unfortunately I'm a single mother so I cannot afford to have them dig deeper to fully diag. I only could justify supporting him through treatment until they told me euthanasia was in his best interest. I wonder due to the less than 24hrs from time of showing any abnormal signs (less than 8hrs of signs for anything more severe than an upset stomach) and how they thought he would be fine and then not long later called asking me to go back and spend time with him since he doesn't have much longer. We do not have any other animals here, however my son was with his father earlier Saturday and they do have goats, pigs, chickens, etc and my dog was my service dog so he did go to different places that could have exposed him to unknown viruses/bacteria. I am so devastated and just want to know why he was taken at such a young age. Prior to this he had no health concerns other than some slight food sensitivity.

I am so sorry to hear about your dog;  it must have been an awful shock.  Dogs can get pneumonia caused by a large range of agents including bacteria, viruses, fungi, parasites (lungworm / heartworm), even allergy or inhalation of fumes or gas.  Those with some underlying illness or weakness are less likely to withstand infection, even if the signs are not evident on the outside.  The same is true of the human world;  sadly, in general, ill and older people sucumb much more readily.  They have a weaker capacity for immune response.  However, I suspect that there will be times when the causative agent is simply very strong or present in very large doses.  We are unable to interpret radiographs here because we are not your vets;  however, when you feel ready, we would recommend talking to your vet about the case because the radiographs might have given a lot of information and what followed might have provided more clues.  You must have questions like:  Why my dog?  Why now?  What was most likely?  Was the correct treatment given?  Vets should understand that stressed and grieving owners do no take every medical detail in at the time, so there is frequently huge benefit in talking over what happened. Failing this, the vets may refer the case to a radiographer , pathologist or some other second-opinion vet in order to help you.  In the interests of professional courtesy, this is usually done with a formal referral so that the person reviewing the case has the same information available to them as the first vet did.  Finally, there is a super vet called Shailen Jasseni, who founded the Ralph Site.  They are an internet-searchable charity that offer support with pet bereavement such as you may be unable to find elsewhere.   I hope that something in there is useful - we would be interested to know if you find out any more and feel like telling us.  Aside from that, we wish you all the best in the future.