Not sure if this is or could be the same thing, but a couple of years ago, a 6 month old nosey be female of mine all of a sudden was black from her hips down and she was having a hard time using her back limbs. I immediately took her in to the vet and he did a scan on her, which showed a large mass in the abdomen area so we both assumed it was some sort of blockage. I gave her lactulose for a while per the vet and even though she was drinking lots and eating, she was pooping but everything seemed normal. She passed away within 2 weeks and at the necropsy, the vet discovered she had over-active ovaries, which were producing eggs and a big mass of yolk sacks which was what was showing up as a big mass on the scan.
If yours is a female, it is a possibility, however if your little girl has full mobility of her limbs, then perhaps it is just a bruise.