Constipation?

My 4 month old has been having trouble pooping lately. She’s straining, crying in pain, and her poops are coming out solid and as big as a quarter :( poor girl. She’s been on Nutramigen for a few months now because she was colic and gassy. It seemed to have worked fine until a couple of weeks ago. The pediatrician told us to give her apple juice so we’ve been doing that almost everyday. We just tried prune juice. She’s been eating solids for two weeks now. Sweet potato and avocado so far. Maybe I need to switch formulas again? She has an appointment next week