my baby is 1 month 25 days old and was born prematurely at 33 weeks and kept in nicu for almost a month..earlier she was given pre nan and later on nan pro.. while she was having nan pro , she used to poop once in every 2 to 3 days and the poop was olive green in color . later on i switched to another formula milk similac advance stage 1 . its been 4 days iam giving her similac advance , she is pooping daily but the poop is constipated and yellow in color.. she even cries a lot while pooping..plz suggest should i again jump on to nan pro or continue with similac advance to avoid constipation ...or is it normal to constipate some days... suggestions are most welcome..