My toddler is sooo constipated right now. Can anyone help?

I have recently weaned her from milk and started her on wholemilk. She has been straining and pushing all day trying to go potty. I have slowly been mixing more and more and wholemilk into her milk. She has had only wholemilk for the last three days. It has been about that long since she has gone poo poo. She is 17 months old. I have given several warm baths and have put petroleum jelly in her rectum. Today is the first day we've tried babylax and she is still straining and pushing and clearly uncomfortable. She has been squatting in my lap trying to go all day. She strains so hard and then between pushes it's obvious that it is bothering her so much. Any other suggestions would be so helpful. Thank you.