Why did my cat foam at the mouth when I gave her a pill?

Dear Most Esteemed and Knowledgeable Kitties:
We are giving our cat new medication for an infection. She foamed at the mouth when we did it. What makes a cat do this and should we be concerned?
~Sue
Siouxsie: Well, Sue, typically a cat will foam at the mouth when given a pill because most pills are extremely bitter. [...]