Formula allergy?!

My son is just over a month old and I have been exclusivly breastfeeding him since he was born, however I go back to work in a few short weeks and cannot get my supply high enough to store breast milk. So, The past two days we've been slowly introducing formula. Today I noticed my baby has a rash on his chest and face. 😩   We've already set up a doctor appointment for tomorrow morning, but has anyone's baby had a formula allergy, if so what formula were you able to use?