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The title should say it all, but to add to it: she hasn't been seen by a vet for a few years, but last time she was there the vet thought she was in good health and just a, "larger" cat and not fat. Since then she appears to have slowly gotten what appears to be either a very saggy belly or very saggy rear-breasts.
She is spayed and has been since early youth. She quite active for an indoor cat and has a healthy eating habit (e.g. She grazes throughout the day and doesn't overeat). She has no signs of any kind of illness what-so-ever, including any behavioral or activity changes. She can jump quite high despite the extra baggage in the back and is very agile.
Specifically, the baggage is right where he furthermost rear nipples are, which is large why I am having a hard time determining which it is. Also, her other breasts are totally normal.