I am certainly not an expert in this area, however, the sore could be from a few different things. It could have rubbed against sumthin, it could have taken a prey item that was just a little bit too big & stretched its skin just a little bit too far, it could have caught a claw of a mouse on it while eating (they are VERY clumbsy little creatures) or a variety of other things. Without knowing what it looks like or if it is a cut or what, we really have a tough time being able to help you heal it. A pic would help greatly if you have one.