That is the most extreme version of this bug that I've heard (and I am sorry to hear that about your slave, I really hope you had saves and/or working autosaves to reload) but that general bug is part of what escalated the Farm Expansion update to the one that I've been working on now. That is the active issue that I'm working on and will be fixed.
My files are all over the place with the new update so I haven't tested this specific code yet but this should limit the stress that slaves can get from Lactation (dramatically limit it). If you're comfortable with editing files, open up expansion.gd in Notepad++ or something and search (Ctrl+F) for dailyLactation. Copy/Paste over everything from #Pressure Stress and Swelling to #Keep the Secret with the following. Hopefully that'll let you keep playing on that save (ideally from a day or two before it went haywire) without that issue.
#Pressure Stress and Swelling pressure = person.lactating.milkstorage - person.lactating.milkmax pressure = clamp(pressure, -10, 10) if pressure > 0: #Chance at Swelling if globals.titssizearray.find(person.titssize)*3 < pressure && rand_range(0,100) <= globals.expansion.chancelactationincreasetits + pressure: if globals.titssizearray.back() != person.titssize: text += "$name's "+ str(person.titssize) +" "+str(nameTits())+ " were so filled and full of pressure that $his body could only handle it by "+str(nameStretching()) person.titssize = globals.titssizearray[globals.titssizearray.find(person.titssize)+1] var hpdamage = round(rand_range(pressure,pressure*2.5)) text += " to "+ str(person.titssize) +".\nThis caused damage to $his health. [color=red]" +str(hpdamage)+ " Health Lost[color]" #Inflict Damage. They won't die from it though. if person.health - hpdamage <= 0: person.health = 1 if person.energy - hpdamage >= 0: person.energy -= hpdamage*2 else: person.energy = 0 text += "Due to $his extremely poor health condition, $his energy has been drastically reduced as well by the incident.\n[color=red]" +str(hpdamage*2)+ "Energy Lost[color]\n" else: text += "\n" person.health -= hpdamage #Apply Pressure Stress if globals.fetishopinion.find(person.fetish.lactation) >= 3 || person.traits.has('Masochist'): person.lust += pressure text += "$name's " +str(nameTits())+ " are so "+str(nameStretched())+"that $he would normally feel incredibly stressed by it. Instead, $he is simply [color=green]turned on[/color] by the pain of $his swollen " +str(nameTits())+ ".\n$He gained [color=red]"+str(pressure)+ " Lust[/color]\n" elif globals.expansion.settings.lactationstressenabled == true: person.stress += pressure text += "$name's " +str(nameTits())+ " are so "+str(nameStretched())+" that $he constantly feels the pain from $his achy " +str(nameTits())+ ".\n$He gained [color=red]"+str(pressure)+ " Stress[/color]\n" person.lactating.pressure = pressure #Keep the Secret (if Possible)