Is my Jack Russell Terrier's frequent leg cramping serious?

The most common reason for lameness of a back leg in a small breed dog is a luxating patella - a knee cap that does not stay in place. Without examining him myself this is my best guess. I believe this would be the most likely issue and should be easy for your vet to diagnose. If it is getting worse then he probably needs surgery to correct it. The photos do not show anything outwardly abnormal. ( https://vcahospitals.com/know-your-pet/luxating-patella-in-dogs )

It is difficult to know how concerning this is. The main concern would be pain causing him to tense up the way he did. Pain can come in the form of a pinched nerve, a bad back or a compressed disc. Less common sources of pain would be pancreatic or intestinal pain. Is there a potential that he had a seizure? I recommend to play it safe and have him examined by his veterinarian. The doctor will check him for back or neck issues as well as palpate his abdomen. This is the best way to make sure he is comfortable. I hope this helps.

What you're describing sounds like a condition called a luxating patella. This is when the knee cap slides in and out of its groove. This condition overtime can lead to arthritis and potential he predispose a dog to tearing a ligament in the knee. Your veterinarian maybe to do an orthopedic exam and tell you if this is the case.

It is unlikely that anything can be done especially in a dog which has finished growing but a review by an orthopedic specialist may be worth considering to assess for discomfort and management options. This is nit uncommon in jack Russell and many other small breeds such as shih tzu types

Hello. This sounds very suspicious of a seizure. The average age of onset of seizure activity for epileptic dogs is from 2-4 years of age. Since this is a first time occurrence...or the first that you have witnessed....you may have to wait and see if it happens again. It may help your veterinarian if you can video the episode if it happens again. It is always a good idea to have your veterinarian examine her to make sure there are no other problems and discuss the possibility of seizures. Thank you for using Petco Pet Education Center, formerly Petcoach.

There are some documented cases of pets doing these involuntary muscle jerks during rest which has been characterized as being similar to "Restless leg syndrome" in people. This condition is still being studied in dogs, and the current treatment for this is Pregabalin or Diazepam prescribed by your veterinarian.

Thanks for using Petco Pet Education Center, formerly Petcoach! Sorry to hear that Jack does not feel well. These could all be signs of discomfort or pain. However, those signs are rather non-specific and while I agree with you that something is likely wrong with him, further work-up would be needed to know what is causing him to be unwell. I recommend that you have him evaluated by your veterinarian for further assessment. Likely diagnostics include bloodwork and radiographs if he appears to be in discomfort. Good luck and I hope he does well!