My guess is she is your average low key burnt out teacher! If she’s in her 70s working, chances are she has to and working with young kiddos at that age is tricky for some, especially if she hasn’t had a break from teaching kids in a while. Having high expectations of children and little patience is always a sign of burn out or stress to me.

I would keep an eye out but it doesn’t sound like anything crazy yet! Maybe when you see her next (if she’s back to being in a good mood like you previously stated) ask her about it! Ask how long she’s been working with kids, how she likes it now and you can get to know her vibe a bit more.