We had the same issue a few days ago where we had to make a doctors appointment. We pushed plenty of fruits and prune pouches. She pooped a huge hard poop screamed and cried doing so. Some babies are slower transitioning to whole milk so doctor suggested putting one once of whole milk into a breast milk, formula bottle. Of course if it’s formula mix that first and then add whole milk. Apple sauce is a major source for constipation and we had no idea. Miralax helps we where suggested to put 1 tea spoon in morning and one a night for one week and 1 tablespoons spoon a day for the 2nd week. Although she said we would know what was best and should see how she does and if she just needed it for a few days.
If natural methods like juice or prunes doesn’t work there is a laxative for baby’s 3 months and up called pedia lax I used on my daughter because she hadn’t had a real poop ok like 2 1/2 days
If you want fast relief, you can add miralax to her milk or some juice. But increase water intake and fiber. Food pouches with chia seeds or flax work well. Or you can just buy these and add them to food. Pear always works well for my little guy as well. My son had issues with constipation when he started whole milk, too, and all I did to help was just not give him all day. Now I only give him milk for breakfast and bedtime, and then he drinks water throughout the day. It's helped so much, and he doesn't get constipated unless I give only milk.