How to help a constipated baby?

Help!! My 1.5 year old is sooo constipated that she sweats and screams until she’s done. She will not drink anything but her milk which is lactose free because she has an allergy I’ve tried to give her apple juice, water, googling different things and she refuses most of them and has barely been eating I will be calling her doctor in the morning but I need suggestions on how to help her until I can get her into the doctor I feel so bad :disappointed: