when i add cooled melted butter to my buttermilk (when making pancakes) it starts to get curdled looking. why??

Because at temperatures below the melting point of butter (around 90F) the butterfat changes from a liquid state into a solid and the bits of solid fat disperse, making the mixture look lumpy and curdled. If you pour melted butter into any cool liquid--buttermilk, milk, water--you'll get a similar effect.