If you are seeing a little redness after elimination it may be her anal glands that are bothering her. Impacted and or infected anal glands can be very painful and some dogs react more then others. Some dogs are so painful that they refuse to walk and will scream when touched.
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You can try expressing the anal glands yourself.Here is a video that will show you how. You will only be able to partially express them and an internal expression may need to be done by your vet. In cases of infection oral antibiotics may be needed.
It is also possible what you are seeing is due to an injury in the back or neck Injuries can happen rather quickly and often with out the owner knowing. Moving to quickly slipping when running rough play or something as simple as jumping on or off of furniture has been known to cause such injuries While this is more common in small breeds like Dachshund , pugs, corgies, basset hounds etc it can occur in larger breeds. Dogs that are overweight are more likely to have back problems than leaner dogs.
Some back injuries involve simple muscle soreness. However more serious problems such as , disc injury or degenerative disc disease, cervical disc disease may be the problem. This can cause extreme pain, loss of muscle control, nerve damage and paralysis. A dog that has suffered a disc injury may show some of the following symptoms yelping when handled, lethargy, shivering, whimpering, loss of appetite, trouble urinating or defecating, reluctance to climb steps, lay down, get up or jump, may be unable to move, paralysis of the front or back legs or limping may also be seen.
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Treatment will depend on the severity of the problem. Dogs with mild injuries may be treated with steroid injections anti-inflammatory medications and rest. Pain medication may also be needed Surgery may be recommended if the damage is severe and does not respond to other non evasive procedures.
He really should be seen by a vet as soon as possible until you can have him/her seen do not allow any jumping, running, playing. Prevent all walking if possible only up for potty breaks and if possible carry them out to go When lifting be sure to use both hands, supporting him from underneath the rib cage and the back legs.
If he is still painful tomorrow I would suggest having him seen by your vet.
I hope he does ok soon, please reply back if you have additional question or concerns,