Weight Loss Worried about my baby not eating

I have had a little female Aby for about 4 weeks in the same cage with 2 adult females. She was about 2 months when I got her. I weighed her the day I got her and she was about 500 grams. She seemed to be very shy at first and didn't each much although she seemed to get along fairly well with the other two. I weighed her every day and she varied between 500 and 520 grams for almost 2 weeks but finally she seemed to come out of her shell and started eating a bit more, though not that much hay. During the 3rd week, she got up to 540 grams but still seemed very small. Finally last weekend she seemed to be eating even less than usual and was a bit lethargic. I weighed her and she was 450 grams. I took her to a vet who seemed to know a lot about GPs and said she had previously had some of her own. She tested her stool and said she had a lot of bad bacteria (Coccidia and gram negative) in her intestinal tract and put her on antibiotics and said no veggies for several days. She seems a lot perkier now but still has only gained back about 10 grams and is still way below what she weighed nearly a month ago. I don't know what else to do for her. Anyone ever experienced something like this?