Hi Lisa
It sounds like the little one has a good home with you.
Has she seen a vet? I always take my kittens to be registered with my vet and have a checkup. How far is she thorugh her course of innoculations?
I has the same probem with Mitzi, a Siamese/Bengal cross kitten, when I got her. If I were you I would:
1. Monitor her progress by weighing her. I used to use elecronic kitchen scales because they allow you to measure grams or ounces.
2. Find a good quality kitten food that seems to suit her and stick to it, not even changing the variety, for a couple of months.
3. Lacto B would be a good idea as it would repopulate her gut with 'good' bacteria that have been killed by the antibiotics. Other possibilities are to go to a vet and get Pro-Kaolin. Another alternative is goats milk live yoghurt (not cows milk yogurt). This is available in he UK in at least two of the big supermarkets.
I'd give up on the idea of registering her if I were you.
I have only started to feed grain free lately. Mine are now on Orijen dried (they prefer the 6fish but it is very rich) with a small feed of Applaws canned in the evenings. However, Mitzi's problem was solved feeding her Hills kitten pouches (Ocean Fish variety), which wouold not have been mine first choice but it worked!