Thanks crumbilina... but it was my fault, for real. I couldn't even tell in here before cause i'm so ashamed of myself.
I shouldn't have brought him with me in such a big journey (that day when we went to the vet, we travelled for several hours). He used to travel with me all the time before all this, so i didn't even think about how fragile he was... And when we were on the vet consult Fivela seemed really stressed. We thought it was because of the cats, he's so afraid even of the smell of them and there were many that day. The vet asked if i wanted to give him some oxigen. I said that would only make him more stressed, and that he'd calm down once he was in the car. But later we realized that it wasn't stress... and that oxigen would actually have saved his life. I made the wrong call, and it killed him. I'll never forgive myself and i miss him so much... more than some people.
Time will help, i know. But right now i just feel guilty. My mistake cost me my most precious baby <3