The medicine reacted with the bacteria in the water column. Replace carbon and do 50% water changes for 3 days. What your fish has is a pustule that is from poor water quality/stress. It should clear after several water changes. Continue water changes weekly until infection clears. Before you medicate a tank do a water change(large scale). Do them several days in a row and see if fish's immune system cures itself. If not then use a QT tank at 10g size and combat unknown infection with Marycyn I, Marycn II, Fungal med, and parasite med at one time being sure not to overdose and be prepared to do large water change if fish seems seriously distressed. A 5g bucket and airstone can also serve as a QT tank.(just don't exceed the one tablet dose per med in the bucket). Medicating in the display tank is dangerous(damage to bacterial colonies) and only needed when fish can not be removed or total tank inhabitants are infected. QT tank requires less medicine and is more easily controlled. A ectoparasite that would form a pustule that size would be visible like a anchor worm. I do NOT believe a parasite is the issue.
The bacterium that causes your condition is Septicima. It is easily combatted thru water changes. If problems with bacterial infections continue then the use of a UV Sterilizer is required to keep water born bacterial counts low.
HTH, Rich