I have a serious dairy allergy as well as a sensitivity to gluten/wheat products. This is caused from 'leaky gut' syndrome where the lining of your gut tears and so the food particles enter the bloodstream where they cannot be digested correctly. Healing the lining would possibly solve these problems- which is not easy & time consuming if you try and do it yourself.
For intolerance type allergies, try Fodmapp diet, which is recommended by medical profession. I have followed this on and off over last few years.