Does changing food cause GI problems?

I changed my girls' food from starter crumbles to layer pellets 2 days ago. I put the pellets in with the last of their crumbles so they would mix and transition fairly easily. They're eating them fine, drinking normally, laying like champs, but someone has loose stools. I just noticed it this afternoon while I was cleaning out the run. I clean the run/poop board daily. Everything was normal yesterday. The poop isn't watery, it isn't bloody. It's brown and looks like it was not really formed and it kind of splashed when it hit the ground. I didn't notice anything different on the poop board, so they were good overnight. On Monday they all got their second dose of fenbendazole. I just did a preventative worming because I'd seen wild turkeys in our yard. Prior to the fenbendazole I do know that my RIR would have loose stools that were almost watery at times. That was another reason that I wormed them. For the past week+ her poop has been perfect, just like the other girls'. I'm kicking myself for changing their food so close to the dose of wormer. I should have known better than to change something up, but their crumbles were running out. I can take pics of the poop tomorrow, I just wanted to get the question out there in case anyone had some insight.
Thank you!!