You can get it checked, I told my GP about Matilda having 3-5 runnier poos a day after introducing milk & I had to get a few poo samples and got them tested for lactose intolerance. Hers was a mild form, but basically everytime she had over 50mls of whole milk (not cooked in a dish, just cup) she would have 3-5 sloppy poos the next day.