This MC reaction is logical, she literally got betrayed by everyone she trusted and murdered, and now is on the side she fought for a long time, got a love confession from a being she doesn't understand and barely knows.
The demon lord did share his true name with her, but all this stuff happening suddendly is rightfully only confusing her.
Had him asked her to serve them loyally it would have worked better.
Her heart and mind didn't have the time to heal, mistrust is normal, if anything it would be a bad developmnet if she said yes, and it can still be if it happens too fast.