I could see it being a burn. But not a bite, a nickle sized wound is pretty big for a bite. Was there any red or blood that would make you think this was a bite? Where the scales all intact without punctures? What is you set up with the heat pad. Have you put a temp probe on it to get the actual temp? And your snake isnt sitting directly on the heat source I hope. There needs to be something between them other than the glass or depending on your setup.