My dog almost died PLEASE READ

Today was the scariest day of my life. I got home and my eight-year-old. Shih Tzu was excited and greeted me like she normally does every day. She then went over to the couch, for my husband was proceeded to lay down, yipe a few times then made a sound like whooooo that faded out to nothing and her body went completely limp. I thought she was dead. My husband carried her outside and proceeded to hit her back over and over because we weren’t sure if she was chocking. For about 20 seconds until she finally regained consciousness. We rushed her to the car and I noticed her tongue was purple so I don’t think she was breathing during that time. We took her to the emergency vet for the full work up full body x-ray bloodwork and ultrasound of her heart and abdomen. They found nothing normal and said she was completely fine. They referred us to a cardiologist saying that it could’ve been a heart episode, but they were not certain. Has anyone had this happen and if so, what was the diagnosis. Or does anybody have any idea what the diagnosis could be? I am so scared. I was still shaking. I thought I lost her. I really need answers so I can feel better.