The more I think about this, the more questions I come up with. Have there been any injuries to that area? Is the fur falling out or thinning in that area? Is the area hot to the touch?
I keep coming up with more things to ask, but really, this is what it comes down to:
Either there's blood building up under the skin (a hematoma) or an abscess, or some icky infection that's growing under the skin.
This poor baby should see a vet ASAP.