K.M.
My son had the same problem (he was 3 weeks early as well). For us, our son had a problem when he had too much sugar or too much juice. As soon as we cut back on the juice, he did much better. Also, we gave him chewable acidophilis pills and that made a huge difference.
I agree that it could be the pediasure . . . does your daughter have any problems with milk??
Good luck, I hope she's doing better soon.
K.