Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian. I am so sorry to hear about your little dog's back leg weakness, and incoordination.
Since she is struggling to pick up her rear legs rather then carrying one or the other, and has seemingly lost her to walk normally I am leaning toward some sort of nerve damage. She may be falling more on her right side as the nerve damage/pressure is worse on that side so she is weaker on that side.
One possible cause for her symptoms would be an intervertebral disc(s) (cushions between the bony vertebrae) that are out of place or placing pressure on her spinal cord and the nerves to her rear legs or even a mass in or around the spinal cord.
Another possibility is a condition called FCE, fibrocatilagenous emboli, where a chunk of cartilage breaks off and lodges in the blood vessels that supply the spinal nerve roots. It is very painful initially as blood supply to tissue is blocked off. The pain only lasts a short time, less than a few hours to a day, but the weakness from the nerve damage it causes it can last for weeks or in rare cases is permanent. These usually don't get progressively worse so if your pup's symptoms have worsened this would be less likely.
Another possibility, though less likely as we usually see it in big dogs, is a disease process that affects one or both of the rear legs called lumbosacral stenosis (LSS).
It can have many of the same symptoms as a FCE as it causes neurologic symptoms too. It is caused by weak spinal ligaments that allow the bones in the spinal column to move and place pressure on the spinal cord or it can be due to
inflammation of the ligaments inside the spinal cord canal causing pressure on
the spinal cord leading to loss of function, just like a FCE.
LSS can be painful on and off until the spinal column is stabilized and the pressure is taken off the spinal cord permanently.
My primary concern given her history, breed and age is intervertebral disc disease.
Radiographs to look for a collapsed disc space would be helpful. We cannot always see a collapsed disc space right away, sometimes we need an MRI to see them initially because discs are soft tissue, rather then bones.
If radiographs look fine and you would like a diagnosis then an MRI of her spinal cord in the back of the body will be helpful. You would need a referral to a specialty center for that as general practitioners cannot afford that sort of equipment.
Pain and/or inflammation in both these conditions is controlled with a combination of drugs including nonsteroidal anti-inflammatories like Deramaxx or Rimadyl or steroids as well as Tramadol and/or Gabapentin. If she is experiencing painful muscle spasms then a muscle relaxant like methocarbamol (Robaxin) could be very helpful.
If she has lost feeling completely in her back legs surgery to reduce pressure on her spinal cord is recommended. We can try medical therapy but it is rarely successful once feeling is lost or she becomes paralyzed.
Long term at home you can use omega 3 fatty acids and glucosamines as well to help decrease secondary arthritis. These nutraceuticals help improve cartilage and joint fluid health as well as reduce inflammation.
In the meantime keep her very quiet. The more she moves around the more uncomfortable she will be and the more damage she can do to her spinal cord. Carry her in and outside to eliminate and leash her outdoors so she doesn't hurt himself further. I do not recommend any bandaging as they do not help with spinal disease and should the bandage slip you can cut off circulation to her lower part of her leg.
Here is a link to read more about intervertebral disc disease if you are interested:
Best of luck with your pup, please let me know if you have any further questions.