My dog passed suddenly on Saturday, I did not even know he was ill.

He started coughing at 1am with a cough every 45 minutes. I thought he had a cold. The cough increased until 5am, when I then went into a panic because he was acting like he could not breathe. His heart was beating faster and his tongue was turning purple. My heart was breaking. My truck was in the shop, so I called my sister to pick us up. Bear kept walking to the back door, sitting there panting. I told him to hang on, because aunt Patti was almost there. He was walking, he even went potty outside before she got there. When Patti arrived, I picked him up and by the time we got to her truck we went limp. I lost it, literally lost it. Pink fluid starting running from his nose but he was still alive. We headed to the vet, but sitting there holding him, I noticed foam comming from his nose as well. I knew he was dying. His heart still beating, trying to take a small breaths, until I heard him take his last one and watched his chest stop moving. It was devastating to say the least. The love of my life for the last 15 years, gone. My confusion is….what the hell happened?