This is a difficult problem to tease apart: veterinarians are often challenged to distinguish the physical from the mental/emotional/behavioral problems that we are often presented. Thumper could have had a traumatic episode at the groomer's, or maybe not. I will attempt to break this down into a flow chart of a sort based on physical vs. mental possibilities, but bear in mind that it is possible to have both parts involved.
It was a good decision to have blood work and x-ray films taken to further delineate the problem. Despite your veterinarian's pronouncement that Thumper is healthy, do not overlook the possibility of either an incubating infection (could be viral, bacterial, protozoal) that will manifest over a few days into a full-blown problem, or even smolder for a week or two or three without symptoms until the infection lifts, and Thumper is back to normal. Or, Thumper has an undiagnosed deep, compensated problem that has been present for some time without symptoms (cardiac disease is notorious for this pattern); kidney, liver, neoplastic disease can also manifest themselves after decompensating.
An animal or man can compensate for a severe problem by biological adaptation mechanisms within a narrow range of conditions, an example would be high blood pressure in a kidney patient--the high blood pressure makes the kidney work better for a short time until the heart fails as a consequence of the work to maintain the high pressure. Decompensation can occur after a deeply stressing episode, and the disease is not immediately evident afterward, even with blood work, etc. Monitoring the patient over a two-four week period will help unveil any deep problem, and this could be as simple as monitoring weight gain/loss, appetite, sleep patterns,thirst, bowel and bladder habits, activity level, etc, or as involved as serial blood work, urinalysis, specific blood tests to target organ systems, etc.
Or Thumper may have been utterly terrified by the whole clipping/grooming process. Some dogs are sedated during the grooming, but this should have been mutually agreed upon, and ideally you could have stayed during the process. It can take a few days for some sedatives to totally dissipate. I believe that animals have emotions, but I am limited in how to divine them.
It is entirely possible for dogs to become more fearful as they age, and situations they once playfully accepted now cause significant distress.
Bottom line, continue to monitor Thumper via your veterinarian, ask the groomer in an open ended way if anything untoward happened and if sedation was used. Don't be antagonistic towards your groomer, you want answers so you can enact solutions.
If this is an emotional issue rather than a strict physical issue, then the tincture of time and a safe structured home will eventually reassure your pet.
Life consists in what a man is thinking of all day.
All the best,Clenia Diceus