Take a photo and post it. There's been many cases of anodic corrosion between alloy lugs and carbon tubes. Treks had a problem on some of their older frames where the glue bond weakened and allowed a tiny crack to form. Water would get into this crack and cause anodic corrosion. Not sure if Trek does it, but Specialized used a fibre-glass layer between the carbon-tubing and alloy lugs to prevent anodic corrosion at those joints in case water got in.