Dog sucking down 3 bowls of water, drooling all over - TOTALLY slimmy saliva instantly?

My dog has a clean automatic water bowl outside that has cool water in it, and an automatic feeder for her food. She is old now, and has a harder time moving arond, but hasn't appeared to be sick, although she has recently lost a lot of weight. She loves chicken treats from walgreens, but today wouldn't eat any. I brought her inside and put some of my little dogs food in front of her, which she's never allowed in when they are eating because she usually scarfs it all down completely, but she wouldn't touch it either. So I tried canned dog food, which she NEVER gets, and she wolfed it down completely like her old self - always acting like she is starving around any other food other than her own, (which she does like nad eat, but just greazes on all day since, once again, it is in an automatic feeder). Then she went to the "indoor" water bowl and drank and drank bowl after bowl as is she was dying of thirst. (Three full large size dog bowls). She was very sloppy about it, way more than usual, and then stood there with it just dripping and drooling out of her mouth. I went to wipe it off the floor, thinking it was just water, but it was sooooo slimmy and took tons of paper towels b/c it wouldn't soak in. How could so much of what she just instantly drank in, turn onto so much slime instantly??? She was NOT throwing water up, just drooling. So I put her back outside, which is when she did start throwing up all the canned food she just ate. When she was done, I called her back in but she didn't want to come back in... and she ALWAYS wants in. No signs of diahreah, never seen her throw up before, (although now I wonder since she has dropped sooo much weight so fast). I do live in Az. where we have hot weather already, but she sleeps in the shade all day but can't imagine her just being dehydrated considering her water never empty or too hot, but her nose IS very dry which is unusual for her too. She is now sleeping peacefully inside on the cool tile floor with no further symptoms other than the dry nose. Is the scarfing down water and all the SLIMEY saliva a sign of her dying? I'm assuming the throwing up was either because she scarfed down a 1/2 of a large can of pedigree which she's not used to, or because her she's been not eating - I wouldn't really know since here automatic feeder always has dog food in it, and she just ate too much???