17 month old daughter stops breathing at night

I have an almost 17 month old. There has been a few times that I have put my hand on her chest and she isn't moving any air at all until I shake her or move her. I told this to her doctor and they ordered her to be on an Oximetry Nocturnal Study. While laying on her side her O2 level was between 95% and 97%, but when she would lay on her back it would drop to between 89% and 93%. Her heart rate would drop as well. My question is, is this normal? I can't sleep at night because I am so afraid she is going to quit breathing all together. I haven't heard from her doctor yet, and was hoping for some kind of information to possibly put my mind to rest. I have 2 older children and never experienced this before. Any and all information would be great!!! Thank you ever so much!!