Water params will say a lot here. As Blue pointed out, even if it was an injury, poor water quality can cause infection to set in quickly.
I'll watch for water params to be posted, and will help all I can from there.
Please be careful when selecting a medication for the knife. Watch water params in the quarantine tank if you use one, also. Poor water params can cause medications to have a negative effect on the fish.
Once I know what the water params are, I'll be better able to help identify the exact cause and to help you find a safe medication to use for the problem.