I've never heard of a chick bubbling. I'd separate it in case it's contagious.I'm excited about the chicks but always forget about the worrying. One of my CX is lethargic. Labored breathing. The whole front of it feels "bloated". I was rubbing the chest to feel for blockage and chick blew bubbles. Also daughter said umbilical cord came off ? I'm not home, trying to get pics to share. I got them yesterday and all seemed healthy and no problems till now. Not pasty butt.
I have had chickens 'vomit' Basically they drank too much water on a hot day..... like today. Child picks her up, hits crop (or holds at a bad angle), and splat = water comes out like vomit. It could be something as simple as that ....plus the heat causing the lethargy. It could also be more serious, so I'd separate the chick. The navel usually does fall off at some point. Unless there's bleeding, I wouldn't worry about a navel scab falling off.