I suspect that throughout time this process of "cross nursing" has always been part of the human framework. What nursing mother HASN'T been somewhere without her child, say, in a grocery store, heard another infant start the familiar hunger cry, and had to press her forearms against her nipples to suppress the tingle of a letdown? We're wired to feed babies. If you nursed for a long time that tingle still happens even long after the nursing has stopped.

SRS