New milk

After talking to some people about how often Cullen gets sinus infections and ear infections, they mentioned how some think that cow milk can cause those problems. So I bought him some soy milk to try. He loves it! I bought a half gallon on Friday and it's almost gone! We started mixing in half soy half cow milk so he would not notice taste. Glad he likes it but he's had diarrhea three times today! Could the soy milk have caused it?! Oh no!