Hi. This sounds like a reaction to something like maybe an insect that bit her - or maybe one that just surprised her - and now she's nervous of going on it. Try not to make a big fuss about it, as she will feed off your own anxiety and concern. I'm sure she'll be back to normal in a few days.
Tony's idea sounds probable. Something out there bit her or spooked her in some way ( an ant? a bee? My dog wouldn't go NEAR any place where there were ants!)
It could be something else, something you wouldn't know about -for example if she suddenly got a twinge in her belly or her leg and it happened at a certain spot, she might associate that place with the sensation...just a thought.
Oddly, I am dealing with a cat who is suddenly 'spooked' about the room where her bed and her food is. (The cat isn't mine but belongs to a neighbour, and I am going in twice a day to feed her & socialise with her.)
Anyway, she was refusing to go anywhere near that room. Weeks ago a dog poked its face through her cat door, and she will not let go of that. She hates dogs!
I am working with her but it is a much slower process than it would be with a dog!
So what I do to ry to relax her in that space is I get her cat biscuit box and rattle it, then put one bit of food down just outside. When she looks at me for more, I put another bit down a bit closer to the room....and so on. I have managed to get her to enter her room, and stay for a few minutes using food treats as distractions and enticement to go into her room.
I wonder if you could try to entice your dog outside by using treats?
Or by sitting in that place calmly (maybe take a newspaper or book to read), and rattling a box of biscuits or something so that your dog might venture out there, and start to associate that spot with something positive instead?
Could also be that maybe at one point something with a thorn scratched or poked the dog. She could have also seen a small animal or insect that spooked her. Panting is a common sign in dogs who are having issues with anxiety, my friend had a dog who would pant and hide during thunder storms. Hope this helps!