Could this be bad bruising?It sounds maybe like bruising. Maybe she had some kind of accident or fall, or was hit by something. Best to get it seen by a vet, just in case there are any underlying injuries, or any possible injuries anywhere else that you may not notice. Dogs can be incredibly brave, and hide discomfort sometimes. And sometimes the real pain of bruising like that isn't felt until maybe after she's slept and then gets up. If she has bruised the bone (which is easily done, I've done it myself!) it can feel very tender for weeks and weeks afterwards. If it is a bruise, the fact that it is purple/red would mean it has only just recently happened.
From your description, it sounds like a bruise, but without seeing it, it's impossible to tell, and even then, only a licensed veterinarian can give an actual diagnosis. I would take the dog to the vet just because even if it is only a bruise, you don't know what happened that caused it, and there could be some other unseen injuries that your vet would be able to find by palpating her abdomen and moving her legs around to see if there are any painful areas that need further examination.
Ghilly