How do I know if my daughter's belly is taking the whole milk alright? she keeps having diaherra really bad, too fhe…

How do I know if my daughter's belly is taking the whole milk alright? she keeps having diaherra really bad, too fhe point she has a very bad diaper rash! I'm not sure if she may need a different milk or if tbis is just her adjusting to whole milk? I have been doing half formula half whole milk since friday