It means she's encouraging you. Don't think too much about it.. there's not a guy out there that can last for more than 5 minutes the most. It might seem longer to you, but in reality, it's only 5 minutes. If you want, you can have a clock right next to you and see the time go by. But by getting better doesn't mean getting to last longer. Maybe she's getting more comfortable when you go inside her, or maybe she's enjoying it more, and maybe you're spending a longer time pleasing her before you go in, and you are starting to learn what feels good for her.