Why did my Mac and cheese sauce curdle?

It sounds like it was too hot. I agree with the others...next time add the cheese off the heat and slowly! Try it again!

Temperature is key here. Don't let your sauce get too hot!!
Cheese begins to separate at about 160 degrees F. Grate your cheese and melt it into your sauce slowly and on a gentle heat

Not sure what recipe you're using, so it's hard to say. But be sure you've added the cheese off the heat. And more 'aged' cheese has a higher likelihood of breaking down or curdling.

Try making a Bechamel first ( white sauces) then add the cheese !