My first thought is that this is related to her needing more A time. As you said, most are doing around 3h at this age. It's also when most are starting to drop the CN, and when the don't it can really make things wonky. What's she like when you try to push the A times a bit? Do naps suffer at all?
Could definitely be related to reflux, though. Can you see the doc to talk about her meds (if she's on any she might need an adjustment for her weight).
Could also be worth a shot to put her on her belly, if you're ok with that.
I suppose it's possible that that night a few weeks ago made a difference, but I really doubt it. If it was just once time, and your mom was right there for her, and you've been there for every NW since then, I wouldn't think she's really fixated on that. (That's an easy go-to, though, with Mommy guilt!
)
Definitely are a few possibilities here, but I'd start with thinking about her EASY.
What do you think?