Hi Dario,
I'm sorry I have no personal experience of this but I just wanted to send my support your way. I really hope you can get some medical advice quickly and it sounds like you need a new colorectal surgeon because not caring about your patients just isn't good enough.
I've just had a quick search for some info online and read this
"This medicine may cause chemical burns or tissue damage if not used properly. Make sure to remove this medicine quickly after you apply it. Ask your doctor if you have any questions before you use this medicine."
"Call your doctor right away if you notice any of these side effects:
Allergic reaction: Itching or hives, swelling in your face or hands, swelling or tingling in your mouth or throat, chest tightness, trouble breathing
Blistering and peeling of skin where medicine is applied.
Pain, swelling, or reddened skin where medicine is applied."
Source: University of Maryland Medical Center
http://umm.edu/health/medical/drug-notes/notes/potassium-nitratesilver-nitrate-on-the-skin
All that leaves me even more concerned for you. Is there not a local ER where you can seek help? I think that would be appropriate.
All the best