What did my frog die from???

[Ausfrog]

Can any one tell what they think my frog died from? I'm thinking Red leg, but after reading a bit about it i'm not sure.

The redness didn't seem to show that bad until she died, there was no bloating or convulsions. I just found her sitting on the ground looking a bit dry and when i picked her up she was rather stiffish, but not dead. The most noticeable thing was the large dark mark on the back.

See photos attached, any help would be appreciated.

Note: all other frogs seem healthy, good colour, moist skin and alert.