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Hello and thank you for using this site to help you. Your intuition is likely correct. Generally dogs hide when they don't feel well and we see this most often with dogs that have Intervertebral Disc Disease (IVDD), which is what you have described that the dog went through last year. I would suspect that is what you are seeing happening again.
I would give your Vet a call and see if he wants him back on the same regimen of medication. Until you can see your doctor, you can try a baby aspirin given with food. But do let your Vet know that you have used it because use of other prescribed meds may have to wait until the aspirin leaves the body,
I am going to direct you to a link that goes over IVDD and it will tell you the symptoms and treatment.
lower spine http://www.petplace.com/dogs/intervertebral-disc-disease-thoracolumbar-area-in-dogs/page1.aspx