You are distressed because it is a red flag.
She feels jealousy (whether is the jealousy of missing an activity where you both can be sharing an experience or jealousy that you might attract the other gender's attention is also important, but not as much as the subtext).
Form her side:
- Your WW has expressed an unfulfilled feeling in your relationship (I suppose you are trying to R) caused by an action you performed.
- This unfulfilled emotion could represent an unfulfilled need, a frustration, or even a sense of guilt or detachment or even 'punishment' from your side she feels or interpret you are inflicting her because of her betrayal.
- Emotions of unfulfillment may becomes justification to soothe the guilt (if she is advanced) or shame (if she is early in the process) for her betrayal, and if stacking up may become a slippery slope towards relapse.
From your side:
- You likely read subconsciously those feeling from her.
- Your nerve system remembers what she did the last time she did not felt "seen" or "fulfilled" by your relationship
- You might have done it innocently or not, aware or not. Turns out this gesture (normal and nice in any other context) stirred you both up and this scares and unsettles you.
You may want to evaluate what was unsettling you ( Maybe something else from what I listed, but you need to look inside), the word "Jealous" is probably a simulacrum of the root fears, you can feel it but you cannot see with clarity what is the disturbing subtext.
And once you see the root, you may want to evaluate if you want to leave this hanging or talk it out with her in a way that will calm both your serve systems.
The unspoken emotions might feel easier to sweep under the rug (for fear of conflict) but they will linger, even if it seems like a tiny and irrelevant thing (it is in a normal scenario, but is very relevant for you both at this moment in time) it is the tiny and 'irrelevant' things that cumulate, build resentment and may push a WS or a BS down the destructive path in the long term.
Do not underestimate what you feel or she feels when it hits you like that, no matter how small the matter seems, if you both feel, is your inner self talking and is important if you are trying to rebuild.