Could be a couple things going on here.
1. It's possible that one of the other Sims was unhappy and triggered her moodlet, but they're fine now and her moodlet just hasn't timed out yet.
2. Looks like the toddlers are asleep. Hidden under that sleep moodlet, is one or both in the red? Maybe she's still picking that up.
3. Do you know who she has sentiments with? Is there maybe a best friend outside the household she has a sentiment with, and that Sim has maybe spawned in to walk by as one of the random background Sims?
That said, every once in a while my Sims seem to get stuck in a negative feedback loop. One Sim hammers their thumb fixing the toilet or something, and then the other gets the shared sadness moodlet, and then when they go cry in bed the first Sim gets shared sadness because the second is sad... and I have to send them to separate corners until both sentiments time out so they don't keep retriggering each other! So yes, the sentiment does seem to get bugged occasionally.
I get this a lot, particular with families and it can be set off by anything. Like @PipMenace says the other Sim can get over their issues but their loved one has the moodlet for a while. I often get it just because a Sim really needs a shower and is uncomfortable say if they've been working out and feel grungy. It can be because they've just got out of bed really hungry and even after they've solved their need they still have a parent who is over empathizing and sad. It happens more for me with Good or Family Oriented Sims.
The sadness moodlets have needed work for a while. There are too many of them and they are way too buggy and nonsensical.
