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Sounds like she just has a piece of grass stuck in her throat. That happens to my Beagle sometimes if she eats grass. She reverse sneezes randomly sometimes too as does Alice, but if it was right after eating grass and she's never done it before, I'd think the grass probably just tickled her throat - which I'm sure could cause a reverse sneeze too. She's probably sleeping now because Benadryl causes drowziness. If it was my dog, I wouldn't give Benadryl because of one time with something like that - it is a medication and I'd reserve it's use for if your
sure it's allergies (at 7 years old if this hasn't happened before, I'd doubt it is). I wouldn't worry unless there seems to be an onging problem or if she has another symptom.