For dairy re-introduction; clarified butter, also known as ghee is the gentlest form of dairy to start with. It is possible for some to tolerate this from the beginning because all of the whey, lactose and milk proteins have been cooked out. The milk sugars can also encourage yeast growth.
Commercial ghee may contain additives so read your labels and purchase only from a quality health food store. Remember to buy organic. If you are unable to find untainted ghee you can easily make your own, (see recipe below). If ghee is not tolerated now wait a few weeks and try again. If ghee cannot be tolerated at this point then dairy may need to be omitted from the diet permanantly. If all goes well with ghee then next try using organic butter.
RECIPE FOR CLARIFIED BUTTER OR GHEE
Heat desired amount of butter in a small skillet. Once melted the milk sugar (white residue), will separate from the clarified butter (the yellow liquid). Skim off and throw away the white portion. When cool, pour the yellow liquid into a glass storage container with a tight fitting lid and refrigerate.