Pooping HELP please

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My baby has switches formulas 3 times because she keeps throwing them up badly. Ped switched to Emfamil AR and it's been great for spit up BUT it causes her to have a hard time pooping. She will poop but she cries badly and her stomach gets hard . It's like shes straining . If she's constipated does it mean she needs a new milk?? Or something to help her poop easier , Help please