I keep finding skid marks (poop stains) in my girlfriends panties, what do I do or say?

I have actually had this problem before. It's kind of a sensitive thing to bring up to your girl but i eventually found a way to hint at it without embarring her. What i did was i grabbed the shitty underwear, cut out the shitty part of it and dropped it into a cup of milk, after that i stirred it up a bit until it got a chocolate color and i served it to her. As expected she said "hey this milk taste like ****" and i politely replied "Yes honey, I know. That's because its made with all that **** you leave in your panties." And that was the end of that problem.