My infant has been on Alimentum for a few days now, its the ready to use kind, she was originally on?

Nutramigen for almost a month, but the blood in her stool came back, so the doctor suggested Alimentum..anyway, my questions is, my daughter is extreemly gy, and had stopped pooping. she's only had one BM in the last three days, is this normal when switich formula?? will her BMs go back to normal soon?? and will the gas stop? Should I try soy, or maybe lactose-free??