I found skids on my girlfriends underwear and now im completely baffled and disgusted!?

My girlfriend slept over the other night she usually does so once or twice a week. We have been together for a few months now and i was really into her UNTIL i examined some clothes she had left in my hamper after taking a shower. (She usually takes her clothes home but i think she forgot.) Well in the midst of it all i found her underwear with a giant sloppy brown **** stain (still fresh) and now im completely turned off and disgusted and not to mention baffled. I'm thinkin about breaking up with her but Im not sure on what i am going to say. Any advice would be helpful and appreciated! =]