Hello, and welcome!
I'm so sorry Lilly has you worried.
Since she spends time outside, she could have ingested a toxic flower, plant, grass, etc., or if she's a hunter, she might have eaten a rodent or other 'critter' that ingested poison, and she could be poisoned by proxy.
A head trauma is also possible, from being struck by a vehicle or bike, or an evil human and she could have a brain bleed or swelling of the brain, a tumor, had a stroke, etc. I don't mean to alarm you, just to inform, but while she was away from the house, there's no telling what could have happened, so we have to make educated guesses.
It would be advisable to have her evaluated in person by an ER vet center, now, so you can find out what's causing her symptoms and have her treated.
I hope all will be well with your Lilly and please let me know how things are going. Thanks!
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