Anyone ever deal with anaphylaxis with their dog?

I had a strange episode yesterday. My dog was happy, running around in the house, barking and playing, eating normally, etc. Then he throws up 3 times in a row what looks like 3 separate meals. All of them nothing but his normal kibble. He then laid down and didn't want to move. He just looked like he was sick. Then he became unresponsive, shallow breathing, and starring off almost like he was blind and couldn't see anything. I rushed him to the vet where he basically defecated as I was carrying him. No clue why. We looked around for anything he could have eaten, there aren't any bugs in the house, and since he threw up all his food, I went through it to see if there was anything in it. The vet said his temperature dropped and he was pale. He couldn't even stand and just went limp. After four hours at the vet, more vomiting, and treatment including two different drugs and IV fluids, he started to do better. We brought him home last night and he slept all night and woke up this morning about 98% normal. Took him out for his walk so he could go to the bathroom and everything looked normal. He was so hungry, he went to his bowl and started licking it without any food in it. I made some rice and mixed it with some of his kibble(at the vets direction) and he gobbled it up. He went or a followup visit at the vet this morning and he seems fine. The vet also gave be some medication for his stomach, but there is no real answer to what happened to him in the first place. We searched the entire house for anything he could have gotten into, but we can't find anything. No food, cleaners, bugs, anything. We haven't changed his food. We're stumped, but I just don't want it to happen again.