I have a female about the same age that will only take mice. I was trying to get her switched to rats and she was stubborn and started to loose weight as well. Now I plan on waiting until she is full grown to try again, I do not want to stunt her growth and at this point jumbo mice are fine for her.
A note on weights, when do you weigh her? Before/after feedings, that weight loss could just be an empty belly.
It might be worth getting a fecal done by your vet to rule out parasites, especially if this was a pet store snake.