What happens to Clay while he's in the candy shop?

From the text:

I can’t bring myself to look toward the front counter. Not yet. I don’t want to imagine her standing there. At the back of the store, behind a wall of see-through doors, are the refrigerated drinks. And even though I’m not thirsty, I go there. I open one of the doors and take an orange soda, the first plastic bottle I touch. Then I walk to the front of the store and pull out my wallet. A wire rack loaded with candy bars hangs from the front counter. These are the ones Hannah liked. My left eye begins to twitch.
“Is that all?” he asks.
I place the soda on the counter and look down, rubbing my eye. The pain begins somewhere above my eye, but it goes deeper. Behind my eyebrow. A pinching I’ve never felt before.