Perhaps it is made of a hard material, which rubs you too hard. Also, that kind of thing happens to me when I sweat a lot, and the places where the skin contacts the most with clothes or something like that gets a rash or an irritation. Might be something similar in your case. What you could try to do, is to take it off whenever you are not using it, although that would be quite inconvenient, but it would decrease the chances of that irritation to take place.