If it's unusual for her, she probably pulled a muscle in her back end. It can also be a sign of abdomen pain. Is she eating normally and able to jump up on the couch?

Some dogs move like this all the time. In sled dogs, this is called a "crabbing" gait. It's generally seen in dogs with poor conformation--like my Pingo who crabs as soon as she's in harness! It's not a terribly effective pulling gait so most mushers don't like crabbing dogs. Pingo has such horrible bone structure that she has no choice but to crab. It works for her. Her son, Sundin, does it as well, but to a lesser degree. He has longer, straighter legs!