It is very difficult when you wake, you sense they aren't breathing, when you're barely conscious yourself - and you panic and do the first thing you can think of.
It's hard to know when they stopped, whether they are able to begin again on their own, or what.
I wouldn't thump her on the back unless the blowing doesn't start her breathing again. Also chest compressions are for a stopped heart which is a bit different.
If she really won't breathe then dial 999, and keep giving her quick breaths directly into her mouth until help arrives. Hopefully this will start her again but you cannot be too careful.
Mine stopped doing it around 6 months I think. He only did it a few times; the doctor wasn't interested apart from to tell me not to co sleep (wrong advice imo, as it's been shown to assist with breathing regulation, and I needed to be that close to know when he stopped)
if you google sleep apnoea in babies you'll find various discussions and studies - there are dangers if the infant has life threatening events, as they are called, when breathing stops for long enough for their sats to be reduced and their lips to go blue.
It is pretty common otherwise and they often grow out of it apparently.
Good luck and do go and ask if you want more reassurance from them. Also continue night feeds. I don't think feeding at night increases the risk at all. In fact common sense says they may reduce the likelihood of it going unnoticed, as you will be more alert/awake during the night.