If all you ate is cheese, you'll stay in ketosis. Very few carbs in cheese. It's the calories that are the problem, though. Just a matchbox sized piece can have more than a shake's worth of calories in it.
It's worth getting a few extra Cambridge products for emergencies as it's much better to have an extra shake than to succumb to the munchies.
Easy to say, I know. But don't panic - so long as you didn't move on to other food, you'll be fine.