Ya, sounds like it. Can you get the 12.5%?
In inground pools, soil bacteria can enter the pool through rain water runoff and convert CYA to ammonia. This results in extreme chlorine demand and shows up as very high CCs. So the 5 result is a red flag. It's not impossible to see it in clear water too, just treated a pool that was clear but had an ammonia of 8ppm and had CYA from the Dichlor the pool guy that came a few days before me used to try and fix the problem.
You'll need to test and add chlorine up to every 30 minutes. Don't bother testing the CC during this process to save reagents. Test the FC with the FAS-DPD. When the FC is holding at your shock level for more than 1 hour then you can test CC and see if your making progress. When the ammonia has been almost oxidized you'll see the CC start to drop fairly quickly.
Additionally if you want to confirm you can go to a local petshop and purchase a fish tank ammonia test kit.
Here's an article to test the extreme chlorine demand, another hint if your dealing with ammonia:
http://www.troublefreepool.com/post56824.html#p56824