The Swollen factors take into account their frame size (ie: height), vagina/asshole size, and pregnancy status. If a tiny and/or small race Slave with tiny holes was cum into once and impregnated, it could occur. It may not be a bug, but just to check: What is the race, height, Swollen number from Statistics, and are they pregnant?
Viewing post in Aric's Expansion Rebred
Do you mind uploading a savegame, especially if there's no bugs showing? I'd love to help ya get this resolved. It sounds like she's definitely pregnant, as that's the primary swelling feature for Tits. That level of growth shouldn't happen, it should cap at a potential 1 per day (and have a chance of losing 1 size per day after birth). The swollen feature hasn't had this issue for me, so I'm curious if there's a factor that wasn't there in my games that I may have missed.
Hopefully this works,
https://mega.nz/#!9G4QhY7D!rDQ8YrMto2_qxZnOmUep0SdIX4BxW_RZK3JZfY23ClY