Food Allergies Treatment

so my sweet little 4 month old daughter has what seems to have been a cold for about 5 weeks now. at the beginning, she started off sneezing and coughing which then turned into a very congested nose. i used an aspirator several times which she really loved. (not.) then we started using a vaporizer and the mucus has gone away a lot. but it’s still there and is taking forever to completely go away. it gets worse when the evaporator runs out of water. ENT doctor said he didn’t see anything wrong but did say he say more mucus than he normally does. he had no other suggestions. could she be allergic to milk? she drinks organic formula. thoughts appreciated.