I am sorry to hear that your girl Ruby Tuesday has screamed suddenly several times in the past few days and now seems frightened.
These symptoms certainly indicate pain, but I understand that you have been unable to localize the source.
Has she ever had radiographs of her spine or hips?
If you pinch her toes on her rear feet does she feel it?
Is she lifting her feet and walking as usual or does she seem to be dragging her toes as she walks?
If you flip her rear feet so the topside is down when she is standing does she immediately right them?
It is important to find out whether she is weak as well as painful.
Is she having any trouble holding her eliminations?
It is very possible that she is dysplastic. Symptoms can happen suddenly if a piece of the arthritic changes in her hip breaks off and is free in the joint. If that occurs and she moves just the wrong way it feels like a sharp pain in her hip.
If she is dragging her toes 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. In her case since she has seemed painful repeatedly this seems unlikely.
Rotties and German Shepherds 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.
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.
If those look fine then an MRI of her spinal cord in the back of the body will be helpful.
Pain and inflammation in these conditions is controlled with nonsteroidal anti-inflammatories like Deramaxx or Rimadyl as well as Tramadol and/or Gabapentin.
You can use these with the omega 3's and glucosamines if 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 alone. 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
If she is not responding to cortisone or nonsteroidals then I think that more diagnostic testing should be done. An MRI of her spinal cord will help diagnose intervertebral disc disease or lumbosacral stenosis. Those conditions can be treated surgically.
In the meantime try and keep your pup quiet. With spinal instability or arthritis the more they do, especially jarring activities like running and stairs, the faster the condition can progress.
Please let me know if you have any further questions.