Thank you for the additional information.
I am sorry to hear that one of her pupils is much larger then the other. This is a disorder called anisocoria. Since she has never had this problem before this most likely did occur while she was on the hike. The most common causes are trauma to one of the eyes such as a corneal scratch/ulcer, glaucoma (though usually the eye is also blue), a blood pressure disorder causing the retina to be damaged, trauma to the head, brain, or nerves in the head from an injury, inflammation in the eye called uveitis, or a fever illness in the body.
Dehydration does not typically cause one pupil to be dilated, though a stroke or a neurologic disorder unfortunately can. One of the concerns after a long hike when it is hot is heat stroke which could cause a neurologic disorder if her body temperature became much too high though usually other signs such as excessive panting, vomiting, extreme lethargy, and even labored breathing are seen.
When Anisocoria does come on suddenly like this, I honestly would recommend taking her to a vet as this could potentially be serious especially if she is showing any other symptoms I mentioned above. If she does have any disease process in her eye the vet will be able to treat her with the appropriate medication to prevent any further problems for her.