I don't think she's rejecting her joey. It is a good idea to be prepared just in case she does so please read up on joey rejection.
She is hissing because the baby is nursing and it is a bit painful. Do keep and eye on her and please make sure to give her EXTRA protein while she is nursing. This will help her continue to be able to produce milk for her baby. If she cannot produce milk then she will either reject it, or "not meaning to scare you", she may actually kill it. It is their instinct to do this as it's something they would do in the wild.