39.8 gram Borneo help!

[Cj92]

Hey so I jus went out and bought a Borneo python she is 4 months old from what the seller told me but any way I fed her a fuzzy mouse as that what she is feed on then weighed her and she only weigh 39.8 grams but is still active and alert what do I do and what is the best way to get her back up to weight this is my fistt blood and in the pictures of her that I seen she looked to be a decent size . I didn't want her once I seen her condition but still took her so she will be in better hands can post picture of needed and if I can figure it out