What is wrong with my chihuahua's rectum?

Sounds like an abcessed and ruptured gland. A trip to the vet is definately necessary. These wounds can become septic extremely fast, and such a small breed dog does not stand a chance with a raging infection like that. Google "ruptured gland" in the images tab and see if that is what you are seeing in your dog. You may need to have your parents look or disable the internet filters for obvious reasons.... About the only thing you can do is GENTLY dab the area with hypoallergenic baby wipes, or soft, lint free cloths soaked in tepid water and try to keep her from licking. A small towel wrapped and pinned around her neck like a fat collar should work if you don't have an E collar for her.