Constipated 14 month old

My daughter never had a problem with constipation till she turned one and I switched her to whole milk. And yes I slowly transitioned her into it. So I’ve switched back to formula and she is still very constipated. I’ve tried to give her prune juice and she won’t drink it. She a horrible eater so it’s been hard to get her to eat the foods that might help. I don’t know what else to do. She’s at the point she wakes up crying in the middle of the night cause it bothers her. Anyone have any advice?