Why is the fur around my dog's eyes and mouth staining brown?

If this only lasted for like 15 minutes at most, he could have just ate dirt or something. Was he just outside, is that a possible answer?? If this lasts for very long, GO TO A VETERNARIAN IMMEDIATELY!!!! DO NOT TAKE ANY CHANCES. I'M SURE YOU LOVE YOUR DOG. IT IS VERY POSSIBLE THAT HE COULD BE SICK. I hope you make the right decision. <3<3 Lots of love to your dog, and good luck.