Is my boa too thin

[4Dobermans]

Pepper is about a year and 5 months old. I bought her in January of last year. I feed her every two weeks with large mice. She always eats but I measured her and she is only 31 inches. Did they sell me the wrong type of snake, she was supposed to be a Colombian red tail boa? And is she underweight or growing right?
She sheds about every two months.