Allergic reaction to cow's milk?

Auggie vomited last night, developed a fever and was fatigued all day today.  After a very mucousy poop we decided to call the doctor and started questioning whether we were seeing an allergic reaction to dairy.  These slow reacting allergies can take days to appear, so it is possible that although yesterday was day #3 drinking cow's milk, he could have had a recation to it as we have slowly been increasing the ratio of cow's milk in his "formula".  All dairy is out for now as we wait for him to recover.....:(  we get so bummed out when he's sick because we work so hard to get those calories into his body.