How do I nicely tell my girlfriend...?

How would I even go about telling my girlfriend that when she wears shorty shorts, her backside bounces. It is very disgusting. And nobody else tells her because they're trying to be nice... but it sometimes literally makes me sick to my stomach. I love her, so I don't want to say it in a way that would hurt her feelings for embarr her half to death. I know this is an odd question, so please be nice. (: