I don't think that cats are supposed to have milk/yoghurt because of the lactose content (most cats being lactose intolerant) so this is a cause of runny tummy's.

Saying that though my two LOVE milk - to the point when i am making a cup of tea & Patch see's the milk bottle he will start pleading for some,. They also love yoghurt. I only give them a small bit of milk as a treat every so often or if i notice that they haven't had a movement so use it as a de-constipationer (if that makes sense). I don't get the runny tummy with them for milk though so i "believe" they are the few cats that aren't intolerant.

Try a little bit of yoghurt and see what happens.

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