Milk Allergy or not?

DD is formula fed and this week i started to put whole cows milk on her cereal in the morning. (i also introduced weetbix 2 days ago) since i started giving her the cows milk she has had a runny poo within 2 hours. She seems ok, not upset or anything by it (so i dont think she has tummy pains) but i am confused.
The formula (karicare gold) is made up of cows milk and no probs with that... so today when i was at the shops i had soy milk in my hands and i put it back because i read its not good for bubs under 2. I dont know what to do apart from not give it to her for while and see if it really is the milk. What if it is? Oh it doesnt make sense...:wall: