Why does Katniss stop to purchase bandages and peppermints?

Katniss is injured while going over the top of the electric fence. She goes to the store, buys bandages, and then stops to get some peppermints for her sister. She is trying to hide how bad her injury really is and unwilling to let her mother and sister know she was in the woods. When she gets home, the Peacekeepers are waiting for her, or more likely waiting for her "not" to return.

My mother and Prim can't know I was in the woods. I need to work up some sort of alibi, no matter how thin. Some of the shops in the square are still open, so I go in one and purchase white cloth for bandages. We're running low, anyway. In another, I buy a bag of sweets for Prim.

______________________________________________________________________________________

“Head Peacekeeper Thread sent us with a message for you,” says the woman.

“They've been waiting for hours,” my mother adds.

They've been waiting for me to fail to return. To confirm I got electrocuted by the fence or trapped in the woods so they could take my family in for questioning.