This means the lesion is in her spine between her front legs and back legs (thoracolumbar spine). "Lesion" can mean infection (diskospondylitis), trauma, inflammation (immune mediated inflammation in the meninges), tumor, FCE (fibrocartilaginous embolism), or disk disease. Of all of these, intervertebral disk disease (IVDD) is HANDS DOWN the most likely cause. Immediate treatment is STRICT CONFINEMENT to keep the bulging disk from protruding further as well as anti-inflammatory pain medication to reduce swelling around the disk that is impinging on the cord.
Now, all that being said, if she is continuing to progress in her neurologic signs and becoming more paralyzed/unable to use her back legs, this constitutes an emergency. The best hope for recovery from a ruptured disk is surgery within the first 24-48 hours. Based on what you've told me, you need to take her to your veterinarian TOMORROW for exam and determine if she is already a surgical case.
Here's a good article for you: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=3223
You're vet will re-take radiographs, too, to rule-out other possibilities (like diskospondylitis or trauma). Until you take her in tomorrow, put her in a crate only letting her out to go to the bathroom (and support her with a towel under her belly when you walk her outside). Absolutely NO STAIRS, GETTING ON/OFF FURNITURE, ETC. Crate only.