I think she catches me staring.
Why would you stare at your girlfriend?
I like her ass and I'm attracted to it; she knows exactly how good her ass looks.
While it is good that you are physically attracted to your girlfriend, it isn't appropriate to stare at her if doing so makes her feel uncomfortable. Your girlfriend may feel objectified if you are just focused on her physical appearance. Instead of staring, you should try giving your girlfriend positive compliments.
That's good advice. I like when she compliments my body as well and she knows how to do that well.
I'm glad that you can appreciate how things might seem from her point of view.  Even when you let her know how beautiful she is physically, be sure to also mention how much she means to you as a person.