Hmmm, I'd think pain. Just from experiences
What do you ride her in? (Bit, hack, halter, etc..) there could be soreness or pinching.
Possibly an abscess?
We have the best evil pony mare, dream kids pony. Old, never sketchy, and will walk the pattern or flat get down.
Once had a bit pinch her lip around the barrel, totally blew the pattern.
Once had an abcess that we didn't see, and shed been running fine, do a 360 between barrels.
May be small and seem unrelated, but they'll let ya know.
Just something to rule out.
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