She could be skinny because she is a lanky, growing teenager, or because she isn't getting enough nutrition. Even though she is picking at the grain, make sure she has fresh hay and clean water at all times. The hay really helps keep her rumen healthy, they should always have some to nibble on.
One thing I'd worry about is whether she has a load of worms and/or coccidia. They are two different problems treated with different medications.
Coccidia is a killer of kids especially, and causes failure to thrive by destroying their intestional lining. That means they cannot absorb nutrients, they become underweight, unthrifty, and anemic. Adult goats can usually handle a small load of coccidia but kids cannot.
To figure out if she has a high coccidia load or a worm load (and what types), bring a fresh fecal sample to a livestock vet. Though when I get a new goat (kid or adult), I hit them with a wormer and a coccidiastat while they are still in quarantine. The wormer I give three separate treatments, each ten days apart, then a final one thirty days after the last. The coccidiastat they get for five strait days, then 21 days thereafter until they are over six months old. An adult goat only gets one treatment. Then they only get it if they need it (a fecal test).
FYI, if she is being kept alone, you should really consider getting a second as soon as you can feasibly do so. Goats are herd animals, and humans are not a replacement for the 'herd'. Keeping a goat alone leads to a very stressed out goat.
