Milk allergy???

So I thought we were safe with the milk allergy thing because James has been drinking regular formula for over 3 months now but I am starting to wonder. James had a little formula here and there during the first 6 months of his life and did pretty well with it. He had gas after the first few formula feedings but then was fine. But now I am trying to figure out why he has had a minor cough for the past 3 1/2 months and am thinking it may be a milk allergy. The research I am doing says that the symptoms may not appear for 7-10 days... well, I stopped nursing on October 14th and James had diarrhea for 10 days starting October 29th. About the same time, he developed a cough and clear runny nose. Other possible symptoms include a rash which he has had on his stomach and around his mouth. I am also reading that it is genetic and both my dad and I had a milk allergy as children. So, I'm thinking it may be time to try a different formula to see if we can get rid of his cough!