it sounds like he has columnaris but the lump could be a boil and may eventually erupt. you will need to do more in the way of water changes and get meds to treat this. its a bacterial infection so ask at your pet store what they have for it.
I would also salt the water to .2%, which is 1 level teaspoon per gallon with aquarium salt and then 12 hours later add another teaspoon per gallon to get the .2%.