4month old baby girl always constipated?

My daughter is 4 months and is always constipated. We've tried prune juice and it worked for a while but shr started rejecting it so my doctor recommended apple juice and it didn't work ad quickly or as effective but now it doesn't work st all no matter how much she has. It just seems no matter what we give her she is always constipated and er have to give it to her regulary every day for it to be semi normal. She ha alot of trouble with it and its to the point of crying. I've considered switching formulas but I'm not sure which one to switch to and how to do t. Id call my doctor but they always ask me to come in and im seeing her in a week anyways. Anybody else having this same problem? Please help :(