Lactose intolerance/Milk allergy?

I’ve been transitioning my 1 year old to cows milk the last couple weeks. The last week he’s had diarrhoea everyday! Has anyone else experienced this and could it possibly be intolerance/allergy?

I’ve of course googled symptoms but he isn’t throwing up, isnt bloated and doesn’t have any rashes. The only thing I see that corresponds with his diarrhoea is nappy rash. The first week I transitioned him he had normal solid poos and was absolutely fine! He also used to have milk in his cereal etc before this.

Could it just be a bug/the body adjusting or shall I go get him checked out with a doctor!

Thanks ladies❤️

for reading❤️