NTA. I’ve worked at a grocery store before and they literally told me during my training that if I am on a break or on my lunch I am NOT allowed to help a customer. You can redirect them to someone but otherwise you’re supposed to just go on your break. I can’t remember the reasons why since I don’t work there anymore but I think it has something to do with time management? If you spend 5 minutes of your break helping someone you can’t get those minutes back, so it cuts your break short. Idk if that’s how it is at your store but I think your manager overreacted.

However, if the item she was asking for was literally behind her you could’ve just pointed them out before excusing yourself. Idk people are hard to please. Bring it up to HR if the situation escalates to prevent yourself from being fired.