hard time pooping

my dog is 7 months old and shes been on orijen from the start, i had her on half iams half orijen for a while because i had bought the bag of iams when she had no other food and only walmart is open. so now i am done the bag of iams i am going from half/half to just orijen, but today and yesterday she seems to have a hard time pooping. it takes a lot of effort and some time. she was ok on this food beforehand do you think its just the change over from iams to no iams, or that she needs to drink more water? have you had a dog that had a hard time pooping? maybe its just a one time thing, but my cousins dog had a inestinal block once. he had to get surgery to remove a bunch of bottle caps.this is probably ok because she did poop, though any thoughts on that possiblity? what else can cause a dog to have a hard time? i think im just a worried mama :P