Unexpected death, young Australian Shepherd

I am still keeping most of this "compartmentalized" as i am having trouble accepting the passing of our three year old Aussie.

She was healthy up to Friday last week when i thought she was constipated for a couple days, so when she still was constipated the next day (Saturday) i took her to a new vet that was open on Saturdays (It was Saturday afternoon at 4:30pm), one that is open 24 hrs for emergencies, as the normal veterinarian we went to around here is more of a clinic.

I explained that she was not eating, and though her back end was uncharacteristically now becoming regularly messy, never did i see any poops drop, though i did see her strain several times. They took X-Rays and believed everything should be fine, just to watch her, and they also strongly recommended we get her on Heartgard+ 272mcg Chewable and NexGard Chewable 60.1 - 121 lb (She was 62.3 lbs). She never was on those before (we moved here to Florida from Colorado Springs, where both ticks and mosquitoes were rare).

The 04/25 invoice shows:
Examination General
Total Health With UA 4Dx O&P G
Radiograph and Interpretation
Heartgard+ 272mcg Chewable
NexGard Chewable 60.1-121lb

I brought her home and we gave her a few meals of pumpkin, on Sunday and Monday she was pooping just fine.

Tuesday, in the early afternoon i gave her the two medicines.

Tuesday nite she was drinking water in the extreme. Where she normally would have a doggy bowl that would last her easily throughout the night, in the evening alone she literally drank down more than three, perhaps four, gallons. Though i did not observe it, my wife stated that she was peeing around the house and she could hear "sloshing" in her stomach.

On Wednesday morning i took her back in to the new vet, though she saw a different doctor, as they traded shifts. I explained that we were alarmed by the severe increase in drinking. I was told that the blood and fecal work from the visit a few days ago came back fine. She was still able to walk on her own and jump into and out of the car, though with much less energy. They did multiple tests, and took two more x-rays. She was negative for diabetes, and thought they had indications of pancreatitis. They suggested that she could be "hospitalized" there or we could take her home and self-medicate with several prescriptions, and recommended a very bland diet (boiled chicken and rice). I asked if there was any significant chance that i would be risking her life if i chose to treat her at home, the vet indicated she did not think so, stating that the level of severity was about 5 on a 1 to 10 point scale, and had me schedule a follow-up visit for the next day.

The 04/29 Invoice Shows:
Recheck - FollowUp Examination
Radiograph - 2 views
Pancreatitis Test
Blood Glucose Test Alpha Trak
Cerenia 51-75lbs
Tramadol 50mg Tablets
Carfate Tablets
Omeprazole Capsule 20mg
Cerenia Tablets 60mg-4pack

Once home she was very lethargic; though normally very active, she seemed to not want to move around at all. I presumed they must have given her some medication and that she needed rest.

She was still drinking a lot of water that nite and threw up some of the water, and whimpered throughout the nite. Though we had a follow-up visit scheduled for 2:40pm Thursday, i drove her in to the new vet in the morning. I had to carry her into the car. Breathing was labored and she would occasionally whimper on the ride in.

Once i got her to the vet, the same doctor she saw on Saturday, he carried her in with his arms under her chest and belly. Though it was raining, it seemed to me she was peeing water. He took her into the back and then almost immediately he came out and asked me for permission to do CPR as she stopped breathing. She couldn't be brought back.

Though they offered to send her out to be examined, i declined as that wouldn't bring her back and i was stunned; i couldn't reconcile how she could have passed away. He asked me if it were possible that she could have gotten into any poisons but there is nothing - we have always been very careful to keep anything out of reach since an early episode with a bag of Dove chocolates a couple years ago. We laid her to rest.

I am not so much looking to blame, as much to understand.The only variables other than what she might have had going on internally, if anything, during the second visit to the vet were the HeartGard and NexGard chewables, and the canned pumpkin. Because the mosquito and flea medicines are so widely used, i doubt the connection, but i still must wonder; they were absolutely the only variable other than if some strange condition existed or anything they might have given to her that i was not aware of.

Can anyone, especially a professional, provide inout as to what might be related to the extreme thirst and passing of our dog? As every one in the past has been, she is a family member; I am still blaming myself for not choosing to "hospitalize" her. I would appreciate some help, please-