First, lactation doesn't cause stress in the vanilla game, so I recommend mentioning mods used and checking if the mod creator, probably Aric, has mentioned it.
Second, the vanilla game doesn't have any way to stop lactation. So you would need to edit the save file to stop it.
Third, in the case that you have AricsExpansion, "lactationstops" in expansion.gd needs to be set to true by editing the file. Not sure if it actually works, but a quick scan of the file turned that up.