It really sounds like a dairy sensitivity to me. They can just pop up like that. I would try cutting dairy out of your diet or at least cutting back for a few weeks and see if it makes a difference.
Does your pediatrician have a nurse line you can call? With our doc if we call after hours they will have the on call paged and they will help figure out if you should come in ( and its free)
"Don't compromise yourself. You are all you've got."-Janis Joplin