curdled 1/2 & 1/2 in Keurig-type coffee
The little 1/2 &amp; 1/2 containers are ultra-pasteurized, so that they can be stable at room temperature.  Ultra-pasteurized milk products will not curdle.  This is why you cannot make cheese with them.

It is also possible that the Keurig coffee, being better roasted and sealed, is more acidic, but it is far more likely the UP milk.  You can test this by finding UP 1/2 &amp; 1/2 at your supermarket in a carton, and comparing.

As a side note, I do find it pretty horrifying that you are making coffee, putting cream in it, and reheating it several days later.  Maybe you could consider making a fresh pot?