Well, what the above poster stated. Isolate your hen.
Is there also a runny nostrils to the sneezing?
If so, is there a color to it.
What exactly do you mean by diarrhea ? watery without any solids,
some solids with lots of urrate (white), if you have pictures of the poop post it. If you have pictures of the bird, post it.
It is very difficult to give you answers to questions. Unfortunately a chicken does not have a symptom to disease one on one ratio. They are complex animals and like us a disease my show up with symptoms that are hard to pin to a disease.
You may be dealing with a virus, a bacteria a toxin, it could be ANYTHING !! This is why we as a group, in order to help you need more information and pictures are a blessing to us.