How young (realistically) can SM present itself?

I just brought home a 4-month old cavalier puppy who has been a dream so far...he's just the sweetest little boy. Today, for no reason that I could discern, he screamed in pain (lasted for about 10 seconds) while adjusting his position on the couch. He had one minor yelp about 30 minutes later, but it was much shorter than the first.

Having done a bunch of research about the breed before bringing him home I am, of course, concerned about SM but am I overreacting? He has a serious of vaccinations two days ago, so perhaps he's still feeling the effects of those?

Any advice/feedback would be very much appreciated!

Eight weeks I have been told; I know definitely by 12 weeks on MRI. But cases that young are thankfully pretty rare.

A single episode like this though could have any number of causes. In young puppies, a problem with something like patellas would be more likely, or that the pup caught something, twisted something, etc or perhaps sensitiviety around the vaccination area. If you are concerned, I'd check with your vet to see if they can find any sign of pain reaction generally... I'd probably just watch for anything else that seems odd or if this is repeated.

Molly did the same at Twelve Weeks, even though she wasn't diagnosed with sm until she was 18 months, the reason and cause of her yelping in pain was because of her patellas (bad knees)!