But she and Adam already had a very clear mandate to have lots of sex and lots of children in Genesis 1:28. I don’t get it.

For what it’s worth, Archbishop Ussher suggests that Adam sinned on the tenth day of the first month in Ussher’s chronology, which is the Day of Atonement. The Day of Atonement presumably represents the first sacrifice, which God made by killing animals (from which He made coats of skins in Genesis 3:21) to cover Adam and Eve’s sin. But we have no way of really knowing how long after the creation week the fall happened (though, it would have to have been before Eve first conceived).