I don't know what formula she's on but my daughter was on Gerber from the hospital and I had samples from Gerber and enfamil at home, we gave her the Gerber first and then tried the enfamil and she wouldn't go poop for 2 days and fussed a lot, her poop looked funny too, so we quit giving her the enfamil and put her back on Gerber and she's been great ever since!
Sometimes the amount of iron in formula can constipate baby. If it continues you may want to ask about a lower iron formula.