That's a dewlap. Females grow dewlaps for extra fluff to line their nests with. Typically, unspayed females will have a dewlap. My girl has a large lopsided one.
Her dewlap could be normal but its not shaped like a lot of dewlaps I've seen. I would feel it and just make sure it doesn't feel like there is a abscess or any strange lumps or anything. But its probably normal.
It could be a dewlap, but usually that will wrap more around the whole neck like a double chin. Because of the odd shape, I would be a bit concerned about a possible abscess. What does it feel like? Soft and squishy, or is it firm at all?
Honestly, to me it looks more like it could be an abscess, especially if it is a lump hanging off the lower jaw and isn't really coming off the neck. Though a very oddly shaped dewlap could, I suppose, be a possibility. This is what an abscess on the jawline looks like:
This is more what a dewlap looks like, like a double chin. This is relatively small. There are bigger, but it still wraps around the neck like a poofy pillow.
The only thing that makes me unsure is it's not hard at all.. It's just squishy. The people we got her from were raising her to eat I don't know if she's just fat or if I should have her checked out.
If you only just got her I'd be taking her in for a general wellness check-up anyway, would certainly be worth asking about it. It is an odd shape for a dewlap.