The woman behind the counter was likely not being rude but trying to provide you with a suggestion that could possibly save you money or get you more food for the same price. McDonald's meal combos often cost less than purchasing the individual items separately.
In fast-food culture, it's quite common for cashiers to suggest meal deals when customers order items that are part of a combo. If you felt the tone or manner in which it was suggested was inappropriate, that might have conveyed rudeness, but without knowing the specific details, it's hard to say for sure.
Customer service can be highly variable, and it's always possible that an employee's intention to be helpful can come across as curt or rude, especially during peak hours when they are under a lot of stress. If you felt her behavior was out of line, you might consider providing feedback to the restaurant. But in general, suggesting a meal deal is a normal part of service in fast-food restaurants.