Vomiting, excessive thirst, diarrhea, mild lethargy/odd behavior... upset stomach or crisis? Without knowing any other background, which would you say?
On Tuesday I was upstairs in the bathroom and noticed a crusty spot on the bath mat and what I deduced were pieces of puke that he didn't re-consume. He must have gotten sick during the day.
Wednesday he woke me up at 3am whining for water. He drank 3 full cups. Afterward we went back to bed and he jumped up on top of me and curled up right next to me with his head resting on my shoulder. He's generally not a tactile dog, so this is unusual.
Friday I had off and was in the yard doing bomb duty when I ran across a reasonably fresh pile that was basically liquid. The Spouse got home and we were sitting on the chair talking and Lucky hopped up, sat on my lap and laid himself along my chest. He didn't move until I shifted him so I could take care of dinner. He fell asleep at 8pm.
As it turns out, it was likely just an upset stomach - he ate "something" unidentifiable at the dog park on Sunday and on Thursday he had Ozzie's food for dinner and stole two rawhide chews (he never gets rawhide). But if I didn't know that we probably would have been at the vet Friday afternoon.