Cause of Dog's Death?

My baby Skittles, a 3 year old Chihuahua/Shih Tzu mix, passed away yesterday afternoon. On Saturday, my mother-in-law called him and he came out from lying under the barbecue grill. But, he was limping and even yelped. We thought it was his leg.

Then, on Saturday evening when we came home, I called him and he came and stood up on my legs, front paws by my knees, ears up, letting me pet him. Happy. He was walking around and acting fine, but walking a tiny bit crooked, barely a limp.

The next day, Sunday morning, he yelped with certain movements and arched his back, tilted his head to the side, upward. He did not want to eat or drink. He was walking with more of curved body, stumbling.

Monday morning, he refused to eat or drink, and had the same symptoms, a little more yelping time to time, not consistent. He would not look at me, keeping head to the side. He stayed in his kennel most of day, refused food and water. My son got him out of the kennel in the afternoon and he walked down the hall, but kept his back arched and to the side, like curved. He pooped 2 medium size balls in his kennel, but did it lying down. He couldn't poop standing up. He yelped every now and then, like 45 minutes apart.

I took him to the vet at 2:45 (soonest they could see him from calling Monday). The vet took him out of the kennel, he seemed to have a left teary, watery eye. He let the vet hold him. He weighed 5.3 pounds. The vet felt him, listened to his heart, touched his back paws, lifted them up, to make sure he knew what his feet were for, looked in his ears, eyes, and mouth. Everything looked OK. He walked around, not in circles, just from one door to the other, by me, for a minute off so, but yelped once while doing so. The vet turned Skittles head from side to side, and he yelped. The vet said it was a disc disease and sent him home with anti-inflammatory oral meds. He gave him fluids, meloxicam injection. (Sent him home with same antibiotic.)

Skittles remained the same that evening. The next morning, Skittles just laid in his kennel. I dipped my finger on water and touched his little mouth. He licked his snout, but his eyes opened and closed quickly for about 30 seconds (almost looked like he was having a slight seizure), his leg moved back and forth more than it had been. His tongue came out a tiny bit, he'd peed on himself.

I went to ask my in-laws for help, because I wanted to wash him with his dry shampoo and give him clean bedding. Skittles let my father-in-law get him out of the kennel and we washed him. I gave him like 3 small pieces of canned dog food, he ate it! He even licked the napkin, but couldn't stand up hardly at all. After he ate, his tongue again came out a little. But then went back to normal. My father-in-law gave him his antibiotic and put him back in his kennel, as he didn't look well at all. I wanted to think he was getting a tiny better, only since he ate!

Two and a half hours later he passed away. He was never throwing up or having diarrhea. I looked on his body thoroughly and did not see signs of blood, cuts, nothing. I called the vet's office to let them know he had passed. But, unfortunately they said it costs a lot of money to find out what caused his passing away. I am just hoping that someone out there has experienced something similar with their little guy and could give thoughts so I'll at least know what I could have done differently, other than taking him to a different vet on Saturday when he started limping and not waiting till Tuesday. He was up to date on all vaccinations.

He was normally a happy, little dog, that would run down the street slipping under the gate, playing feisty with his best buddy Scooby, never bit anyone, never aggressive, just excited, and loved to be pet behind his ears and under his chin.