She MAY have swallowed something, like a shard of bone, etc. which is causing these symptoms. Does her breathing sound whistley? Try to look down her throat (flashlight helps). If you see anything stuck there, at least you'll be sure of the cause!
Otherwise:
I can't guess what started this, but...
Sounds like she's dehydrated, which can eventually kill her. If she's hot enough to pant, try to find a cooler spot for her. And really coax with the water. Maybe a spoon. A bit of sugar in it is fine. Offer her favorite or regular food, too, but it's really the water she needs most.
If her breathing becomes shallow and rapid, and her body temperature drops, then she's likely going into dehydration shock, and needs fluid by I.V. very soon.
You and your dog are both mammals, so I'm sure you know she needs your affection, also.