It is not usual for turtles to get bumps like this. Only thing that can occur is tumors. They can gain size of head of cauliflower. They can even grow on your turtle's eyes and make it blind so it is better to take it to vet as soon as possible. Don't try to clean them by yourself as they can hurt your turtle even more. These tumors should be treated soon otherwise they will make it hard for your turtle to swim.
