Sticky stifle/swinging hind leg?

[scats]

Nutmeg has always had a slightly stiff hind leg since she arrived. You’d just notice it occasionally when she first came out of her stable for a few strides, more that she would just walk a bit slowly at first. Most people probably wouldn’t have noticed anything.
No problems at all until this week when she has developed a very sticky/swinging stifle and resting her toe on the ground rather than weight bear. It’s come on so suddenly that I wonder if she’s done something in the field.

Vet is booked for tomorrow.
Any ideas?

 

[Rough and tumble]

Could be locking patella maybe xx

 

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An update for anyone interested. Meg saw the vet but typically she walked out her stable yesterday far better than she had been.
Still, I felt it was a good idea for the vet to see her anyway. She’s still stiff on it and there’s clearly something that’s gone on, but we are hoping it was just a tweak as a result of something she’s done in the field.
She’s got Equioxx tablets for 10 days (won’t eat feed so cant give her powdered bute) and then reassess after that. Today she’s walking normally.