Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very sorry to hear about Roxy.
Has she ever had radiographs of her spine or hips?
If you pinch her toes on her rear feet does she feel it?
If you support her standing and flip her rear feet so the topside is down does she immediately right them?
It is important to find out whether she is too weak and painful to get up or whether she has lost the ability to do so.
Is she able to hold her eliminations and go consciously or has she has lost the ability to control her urine and stool (incontinence)? If she has loss of control she has loss of neurologic function.
If she is dragging her toes when you get her up that can signify neurologic problems, such as an intervertebral disc(s) (cushions between the bony vertebrae) that are out of place or spinal arthritis putting pressure on the spinal cord 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.
Rotties and German Shepherds and other large breed dogs are prone to a disease process that affects 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.
FCE are initially painful but after that it's just a matter of regaining function.
LSS can be painful on and off until the spinal column is stabilized and the pressure is taken off the spinal cord permanently.
Another possibility if she seems not painful is a condition called ascending myelopathy. This is a progressive degeneration of the spinal nerves that begins with incoordination of the rear legs then progresses to loss of urine and stool control (continence). This seems very unlikely with her as her symptoms came on very suddenly according to your history and she was/is painful.
I understand financial constraints but she really needs a veterinary examination as soon as possible. Radiographs to look for a collapsed disc space or arthritis of the spine and hip dysplasia would be helpful. We need to know what the problem is to treat it successfully. An MRI would be helpful too, but I understand that is likely beyond your son financially. If diagnostic testing cannot be done at least a very thorough exam so we localize the problem and then effective medications can be prescribed.
Pain and inflammation in these conditions is controlled with nonsteroidal anti-inflammatories like Deramaxx or Rimadyl as well as Tramadol and/or Gabapentin.
Long term you can use these with the omega 3's and glucosamines if spinal arthritis or hip dysplasia is diagnosed. These nutraceuticals help improve cartilage and joint fluid health as well as reduce inflammation.
I recommend using a combination of a glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an omega 3 fatty acid (like 3V Caps or Derm Caps). I recommend an omega 3 fatty acid dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give her 20mg of EPA per pound of body weight per day. For example an 80 pound dog could take 1600mg of EPA per day. Omega 3's and glucosamine/chondroitins work synergistically and improve cartilage health and joint fluid quality and quantity as well as reducing inflammation. They can take several weeks to see full improvement but some dogs do very well with them. They are available over the counter.
Another option is a product called Duralactin. This is an anti-inflammatory product derived from milk proteins and it also has omega 3 fatty acids incorporated into it which can be very helpful. See this link for further information: http://www.duralactin.com/products_canine.html
In the meantime try and keep your pup quiet. With spinal instability the more they do, the faster the condition can progress.
If your son's regular veterinarian is beyond his means financially now the other option is to contact the local shelter or humane society and see if they run a low cost clinic or know of one in the area.
If not here is a web-page that lists some that may be able to help:
Other sources of help can be found here: http://speakingforspot.com/?p=Financial%20Assistance%20for%20Veterinary%20Care
Please let me know if you have any further questions.