Please help me with what happened with my dog

[ABlanchette]

My 10 year old chihuahua mix was starting to have some coughing about a month and a half ago. I brought her to the vet and they said she had degenerative valve disease on both sides of her heart. They told us she was not in heart failure but could be close to it. They started her on pimobendan, lasix, and enalapril. She was taking her meds easily with apple sauce but 2 days after starting it she was coughing even more than normal and not going outside more than usual. I was worried because of the lasix and got in touch with the vet. They said that was unusual. I lowered the amount of lasix because I was worried that it was harming her when she started refusing to take all of the meds and threw them up if we gave them to her. She started going outside more after that. Yesterday I had a check up appointment for her. My vet wanted to do some blood work and X-rays on her to check everything out. When we got there she was walking around, sniffing everything, and pawing at me to pick her up, the vet told me that she was doing good and the signs of everything that would happen when she went into active failure. They asked to keep her for a few hours to do tests and told me they would call to pick her up. About a half hour later they left a message saying she was ready. I went to get my son off the bus and head there but then she called and said that my dog took a turn for the worse. She said she went to check in her while she was caged. Which I told them my dog had severe anxiety because she is a rescue and they always caged her so she hasn’t been away from us or caged since, and her breathing was bad and her gums pale. We headed over there and she had an oxygen helmet on and panting like crazy. By the time we got there they had given her a mild sedative and 100mg of lasix. Btw she was not supposed to have sedatives because of her heart issue. I tried to get them to take the helmet off but they said they couldn’t because her breathing. They finally took it off for a min tire but then when they went to put it back on she growled at them. She never does this! Not once in 8 years. I noticed her ears were very hot. Idk what happened while I was gone. They told us she would drown in her own fluid if we didn’t put her down so we eventually agreed. I need help with this because she was fine when I left an hour and a half before. The vet confirmed this. They gave her a sedative before the euthanasia and she died just from that. I little fluid came out of her nose but it was heart breaking. We didn’t get a warning. I just don’t feel right with this. I don’t know if it was the meds or if anxiety had a part. Then after we got charged for the X-ray and blood work and the meds( which we didn’t say yes too because I told them she couldn’t have sedatives). Please help me with this because I don’t understand how your pet can be fine when you leave and then have to be put down when you return. Thank you