Thanks for the additional information. It sounds like your dog may have a problem with a disc in the back. Your dog may have injured his back or have a problem with an invertebral disc. This can happen in many different ways including a dog twisting wrong, slipping, or even being jerked by the collar. The discs can also degenerate unrelated to injury. An intervertebral disc that has slipped or ruptured up into the spinal canal causes inflammation of the spinal cord, which in severe cases causes paralyses of the rear legs. In mild cases, you can see dogs avoiding stairs and jumping up. You can read about this here:
Large dog breeds like labradors sometimes suffer from a condition called degenerative myelopathy which can also cause problems with the rear legs and paralyses. You can read about this here:
Often you can not causse the pain by checking the dog over since it is in the spine. Discomfort and pain happen when the dog moves in certain ways. Buffered aspirin can be given to a dog with a dosage of up to 5-10 mg per pound every 12 hours for pain. Keep in mind that a dog's body does not metabolize aspirin in the same way as a human and thus should not be given more than a day or two without contacting your Vet. The aspirin may need to clear your dog’s system before other medications can be given, so keep that in mind if you decide to give aspirin and be sure and tell your vet when your dog is seen. Basically, if you are going to have your dog seen in a day or two, you may want to hold off on the aspirin. Read side effects and precautions here.
You will want to have your dog seen by your Vet to be evaluated and discuss treatment options. In the meantime, I would keep him as inactive as possible to give the disc time to heal a bit. Your vet can give stronger medications and antiinflammatory medication that can help faster. I would still recommend inactivity and leashed walks to avoid reinjury.
Now there are other things that might cause problems jumping up and with stairs. Arthritis might cause some discomfort but she would also be slow to rise in the morning and have discomfort then as well. Anal gland issues might also cause discomfort in those situation if there was a lot of swelling since jumping and stairs would stretch the skin over that area pulling it tight which could cause pain. You should see swelling in the area and probably a foul odor as well.
Pancreatitis can cause pain as well, but I would expect vomiting and diarrhea as well as a loss of appetite if she was in pain due to that cause. Most dogs with pancreatitis also tend to either arch their back or assume a prayer like position with their back end in the air.
So I do suggest you have your dog seen by your vet. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.http://www.fda.gov/AnimalVeterinary/SafetyHealth/RecallsWithdrawals/