If what you heard was a long blood-curdling scream, that is an involuntary vocalization that dogs can make when they are having a seizure. I had a senior Maltese that did that twice and they never found out the cause and it never happened again. It doesn't mean they are in pain, it just happens involuntarily just like all the other muscle stiffness and jerking in a seizure. It is such a scary sound.
Here is an example:
causes_&_treatment
I think your pup had a seizure, not an injury. I would be trying to rule out liver problems. I'm so sorry.