Chocking while on back

So this has happened a handful of times since she was bron and it scares me every time. My daughter is now 16 weeks and we co-sleep. But she does this while on her back laying on the couch or in bed. While holding her she does it as well. Her eyes get wide and her arms flail, she looks like she's holding her breath but clearly she isn't. It usually ends with me flip if her on her stomach patting her back and then milk comes out her nose and she's crying and I'm freaked out because she is basically drowning. I feel so helpless and I don't know what I can do to help her.