Not to scare you completely, but if an MRI is something you can afford, I'd get one.
We thought our last dog (a 100lb lab/dane mix) had a spinal injury from running into a St. Bernard one day... he went on anti-inflammatories in Nov/Dec and that seemed to do the trick.
Come March he started limping again on a Monday.
Tuesday he was in obvious pain (shaking), and we put him on tramadol right away after a call to the vet. Wednesday still in pain, and not walking well. Got into see the vet on Friday and went straight from my vet to the ER vet for an MRI. At that point, he could barely hold his hind end up. Saturday we said good bye to him and ended his pain after the MRI showed an inoperable tumor in his spine, by then he had lost all ability to move his hind end. He was almost 9 years old.
If we had the MRI done back in Nov/Dec, it might have been operable at that point, we'll never know. We didn't biopsy it, but cancer is a possibility. And the St. Bernard probably had nothing to do with it.
I hope your pup feels better soon!!