Brian,
Sometimes the needle can get contaminated and thus cause an abcess to form at the sight of the injection. The bacteria that causes the abcess are naturally found in dirt and water. If the previous owner had to fish around with the needle in the goats hair at all, it is very likely that it got contaminated and is now abcessing.
The problem you have is not knowing if the abcess is CL related or injection site related. If that were my goat, I would quarantine him from the rest of the herd and into an easy to clean stall or other enclosure for starters. I would then try to draw a sample of the fluid inside the mass with a large gage needle to be tested at the vets or sent to the lab. Finally I would lance the abcess myself in an area that the puss could not contaminate my other goats. In other words, take ALL the precautionary biosecurity measures as possible just in case the test comes back positive. Use tarps, bleach, disposable clothing etc. You don't want the lump to rupture in the midst of the rest of your herd. If you don't feel that you can take those measures, than you can ask your vet to surgically remove the mass without rupturing it thus eliminating the chance for spread.
Good Luck and keep us updated.