Welcome to YT!! My Riddick is very thin also.. I hate feeling his bones, but he is very healthy.. No matter what we feed him he doesn't seem to gain any weight.
I want to try Satin Balls, as I heard they put weight on a dog. Riddick is a mix breed. Yorkie/Bichon (I think there is also maltese in him as the mom didn't look purebred Bichon) He is very active and full of life..
I hope your girl is healthy too!! Maybe just has a fast digestive system..
Again Welcome to YT!!