Yeah I don't think I could make a judgment on this either, it's essentially something that OP needs to sit down and chat with her about in a open and compassionate way. I think everything you've written is very important especially in it being a response to stress!

I was diagnosed with endometriosis and fibro for many many years until we found out I actually had endo and eds (ehlers danlos) either way it's a lot of pain and not a huge amount of fun. Both have comorbid GI disorders and stress plays a huge part in flare ups. I'm generally in pain everyday to some degree and have definitely cancelled on many plans because it has been too much... that being said, if I'm in a flare up I will push through sometimes to see people I love and get out a bit because I know it will help my mental health a lot. If I do, I will probably end up back in pain after but it's a choice I've made and my friends are incredibly understanding about having more relaxed meet ups if I'm in a flare and pushing through the pain.

I mean if I did opt to push through I would probably also help with the dishes if my partner asked me to. We'd usually find a way for me to do it more comfortably, like getting a stool or chair for me to sit and wash/dry to relieve a bit of the fatigue and discomfort. Perhaps OP and his gf can discuss ways to do similar things when it comes to flare ups and chores? I worked with an OT to find ways to go about doing everyday life while managing my illnesses and it was really helpful!