When girls say "Ouch", it is usually a modal particle with a teasing or joking nature, which can express slight surprise, dissatisfaction or finding something interesting. Its specific meaning needs to be judged based on the context and tone of speaking.

If she is expressing surprise or exclamation, you can reply: "What's wrong? What happened?"

If she is reminding you to pay attention to something, you can reply: "Okay, I understand. Thank you for reminding me."

When replying, you can use the same relaxed and humorous tone Respond with a tone of voice, such as: "Haha, what's wrong?" or "Hey, what's good?" This can not only continue the relaxed atmosphere of the conversation, but also express your interest in what she is saying.