Girls, who took antibiotics?

Girls, who took antibiotics? We were on them for 2.5 months. We treated pneumonia, and here's the thing: the baby has stools all in chunks, plus some mucus is there. On IV, we drink lactose-free formula. I think it's because of the antibiotics. Most of all I worry that if the mixture isn't digested, God forbid it won't gain weight, although appetite is good. Sleeps well. Ugh.