My dog cant move her back legs!?

I woke up this morning and my one year old female pit-bull was whining. I thought it was unusual that she was sleeping on the floor (she rarely gets to be in the house so she takes advantage and sneaks on the bed) but I didn't think twice about it. I went to walk down stairs, and she followed. That's when I realized that she was not using her back legs. She army crawled down the stairs, scooted over to the beanbag, tried to jump and didn't make it. I set her on the beanbag and examined her legs. Nothing looks injured. No wounds or swelling, and she was fine yesterday. Please help!