
When my pup was about 6 months old he had wandered into the kitchen to see me after waking from a long sleep. I turned around to face him and nearly had a heart attack at the state of him, his eyelids had drooped down, so much that when they were open you could only see the flesh under the eye, you had to pick up all the dopping to see his eyeballs, which looked sore

He started gettting restless and really salivating loads awsell as acting a little odd (walking like he was drunk and had no energy).
I had him straight to the vets on an emergency appoitment (me in tears - I can't do calm when I think my pets are ill!!) thier first reation was it looked like the effects of poisoning

They examined him and it seemed that his eyes hurt, he wanted to keep them squeezed shut. They give him some pain relief, dropped straight into the eyes and kept him in for a few hours for tests & observation. When I had the call to pick him back up I was so relieved as they said he was nearly completely back to normal, it took a couple of days for his eyes to stop looking sore though.
The conclusion was he must have been stung by somthing (insect)that had caused the reacion (they had checked his eyes and found no scratch/injury to them).
It was very, very frightening and happened all of a sudded. So yes, reactions and eye problems can happen very fast!