Constipated toddler

My 16 month old has been very difficult to feed lately. Ever since she got sick 4 weeks ago she refused to eat or drink water. The only things she’ll eat is jello or fruits but any food she spits out. It’s been very hard trying to feed her she drinks milk not formula but that hasn’t increased her appetite. Her dirty diapers are normal but her poop is very hard and pebble like idk what to do pls help. I will take her to the doctors but I’m the meantime idk what to do.