I'm curious as well. My good friend is experiencing many of the same things you've described with her 3 year old, and I am certain it is an allergy or intolerance though my friend has dismissed my suggestions and will not pursue it further. I think her DS has a dairy allergy tbh. He has had so many suspicious symptoms and I have put the all together but she is getting them looked at and treated separately, and none of them are getting better. But her DS has the same rash you have described around his mouth and it has been there for weeks already, probably over a month even. When she first brought up her concerns and I suspected an allergy, a quick google came up with this link:
http://voices.yahoo.com/dairy-allergy-children-its-not-lactose-intolerance-338392.html?cat=25 which described so much of what she has experienced with her DS (of course I realize this is just google and that not all resources are reliable, so I would suggest looking into it further)
"Symptoms of a dairy allergy can include: rash, eczema, black circles around eyes, or
even swelling around lips, mouth, or throat. Digestive upsets occur in the form of gas, bloating or cramping. The respiratory system can be affected and manifest itself as wheezing, itchy eyes, nasal congestion, and shortness of breath or coughing. Some children even show symptoms of inattentiveness, lethargy,
hyperactivity or bedwetting as a result of dairy allergy."
My friend's DS has experienced pretty much ALL of these.
But she will not listen to me and I know is in denial, which is hard because I see how her DS is being affected but I don't feel like there is anything else I can do.