I’m sorry you lost your Oliver.
The bun mom might feel protective about her kit. She could still be feeding it, even though the baby is probably also quite a bit on its own. She’s still dealing with some mom-hormones and she’s in a new environment that probably doesn’t resemble her old home. If she is eating and otherwise seems healthy, I think maybe you just need to give her some time to settle in? Try to not handle her more than you absolutely have to, give her places to hide (upside-down cardboard box with 2 door holes). She’s most likely not unhappy with you at all, it’s just that moving to a new place is stressful, she has a baby and as you said, her body is already stressed from her having been pregnant, she’s probably had more than one pregnancy.
As for spaying her, it is the best thing you can do for her longterm health. It doesnt have to be done asap though. You can let her recuperate and regain her strenght.