You need to get your puppy checked out by the vet as it could be an infected anal gland or more seriously, a prolapsed rectum.
My Dog Has A Big Red Sack Hanging Out Her Behind What Is This?
I can't really say sir,if only you checked around the (link to pornsite deleted) for this question
Not sure, but sounds scary, probably the only thing you can do to help (Just a guess) is either pray, if you don't do vets, or get to a vet as soon as possible.
I suggest you make an appoitment with your local vet and get her checked out. It could be a tumor or some thing. It could be many things. Did this dog just have puppies, or anything? I would let your vet know what is going on and have her checked out immediately.