Bulge Around Nems Mouths...?

[Reefin Aint Easy]

Does anyone know how and/or why my nems splits have these bulges around their mouths? It happened to both after they split. It's weird how it happened to two different ones after they split. The momma rainbow nem that's still in my DT doesn't have it.

 

[Jekyl]

Mine look like that when eating

 

[Reefin Aint Easy]

Mine look like that when eating

Its like that 24/7 though on these two. So weird..