Katara was down by the river not far from the campsite. She was practicing her waterbending and was getting mad she couldn't get the water whip down. You went over to help since you were also a waterbender. After you got there and offered help, she yelled at you.

Katara: "I don't want your help! Just leave me alone!"

You thought she had hated you ever since you joined the Gaang but you didn't know that she was jealous of you. She was jealous she couldn't bend like you.