Looks like might be a cyst. Feed it sparingly to minimize pressure in the abdomen. If you can get it through a couple molts, it may 'fix' itself.
Looks pretty unique, its likely exceedingly rare.
Well, as cold blood said, that is not exactly a common occurrence. Probably everyone clicks on the thread, zooms in on the opisthosoma to 200% and pushes the back button because they have literally nothing to offer except well wishes (which is precisely what I did, three times). That said, when I zoomed, what it looked like to me was that the insides were on the side instead of, well, inside. Hopefully that's not the case and it's just a cyst. Following Formerphobe's advice is probably the best chance you can give the little one. She knows what she's talking about.