I would like to see a photo if you could im with steve on this it could be anything from an old injury, lymph node, blister, raised scales from mites, flukes and lots of other things so a pic would help. I would like to make a suggestion on the feeding thing the fish you buy at the market like salmon or tilapia is great food but it smells different and some wild caught snakes want take it at first so what you need to do is cut a strip the size you need to feed the snake and then scent it with a minnow like the ones he already ate.