She must be thin. 6 year old Bps are usually near five feet long and really nice and fat. I can't believe the person who owner her before was just feeding mice. This could maybe be part of the reason she's so undersized?
Usually, I have found that adult Bps have a more easy time of striking, coiling and then swallowing prey. My little guys are the worst "killers" (although the rats they eat are already dead, though the snakes don't know this, hehe.) But my big guy never misses, and coils multiple times around each rat he takes.
I would take her to the vet and see if maybe she has some kind of parasites? This could also be part of the cause of such thinness. Whatever happens, just know that you most likely saved her life, if she was in such crappy conditions to develop belly rot. People amaze me.