I can totally relate to how you felt. Delaney quit breathing when she was about 3 months old. Some of the ladies on here might remember our ordeal.
She was laying on the floor while I was playing with her. I noticed her eyes rolling back in her head and I realized she wasn't breathing. I grabbed her, shook her, blew in her face, etc. Nothing worked for about 45 seconds. She turned blue around her nose, lips and chin. Finally she started breathing. She ended up in the hospital for three days while they did a bunch of tests to figure out what was wrong.
They finally determined she had severe acid reflux which basically caused her esophagus to close because of the burning. She is still on Zantac and she was a breathing monitor for three months. It was the type that strapped to her. But it was a problem because sometimes the strap would loosen and the alarm would go off - thus me flying into her room like you - no feet touching the ground.
Thankfully she never had another episode as severe as that first one. A couple of minor ones (they consider it minor if she stopped breathing for 15 seconds or less). She's now off the monitor, but still on Zantac until she is one.
I hope that everything is okay with your DD.