K.E.
I have heard that citrus can cause allergies - in fact, I was told to wait to give my babies citrus after other fruits.
Both of my daughters have the same problem with fruit juice. No one believed me, including more than one pediatrician, but they always had the worst diapers and rashes after a juice box (100% juice). They could eat fruit constantly and not get the same reaction. I have found that as long as I limit their juice intake to one cup that is watered down they are fine. I also don't let them have it (unless it's a party or something) after 2pm because I don't want them to have upset stomachs at night.