Sudden lameness

[insomniaat4am]

Any idea why a collie x terrier we believe would suddenly become lame on a back leg in the night??
We heard him crying when changing position in bed and he's limping badly and really screams when we touch his hip. No vigorous exercise for the last month, just walked on the lead. He's 8 if that's relevant. He's had a sore leg in the past but only after too much exercise and never this severe.

Patella luxation?

Could be anything from a muscle sprain to patella luxation to a cruciate ligament tear

It's just odd that it seems to have happened in his sleep? He's a tiny bit better now he's up and moving. Not limping as obviously but still won't let us touch his hip. He can't get comfy and is just pacing around. Hhhmmm.

If he won't let you touch it then he's obviously in pain - I'd be taking him to the emergency vet ASAP

I'd be at the vet with that. Hope he's ok.

Sorry only just seen this. Randomly by lunch time he was absolutely fine running round like a crazy dog?
Wondering if his hip popped out a bit if he twisted funny and then popped back in?
He's been fine since.
Weird he went from agony to perfectly fine within a few hours? Been looking at hip dysplasia but not sure hips would kind of pop in and out with that?