If the age is correct and she's average size for her age, a mouse hopper is far too small.
Without a scale, you can eyeball the food. The prey item should be about as big around as the snake is wide at her widest part. Don't be fooled by the small head and thin neck relative to the wide part of her body...she'll get the food down if it's about as wide as her widest part. I would guess at least rat pups, maybe weaned rats, but mouse hopper is almost definitely too small.
Wait a few days, make sure all the temps are correct and the environment is all sorted out, get a bigger food item and try again.