my terrapin has a bubble on her neck

by peter james
(stoke on trent,staffs, england)

hi my terrapin has developed a ulcer like bubble on her neck and also her tail looks as though its corroding, she is 4 years old and is eating and swimming around as normal, she does live with another terrapin and they sometimes do snap at 1 and other, but this doesnt look like they have been fighting to cause this, i have changed her food a couple of weeks ago could this be the cause, should i put fin rot treatment in, could you advise please, thanks a lot, regards peter