zamsmooth

I have a 5 mos. choc lab puppy that I am taking to training classes at my local Petsmart. She did great in her puppy class, now we're on to intermediate class. During her training when she was told to sit and wait while I walked away the trainers noticed that she was puffing her cheeks. What does this mean? They said that doberman's do it when they're ready to attack but obviously she wasn't. Was it an anxiety thing?
Also, do labs eat EVERYTHING or is it just her? Any info would be greatly appreciated.
I forgot to ask do you think I should keep a water bottle in her cage at night? She's a thirsty girl to begin with and now with the warmer weather do you think she needs water all night long? Also will this tend to make her have to go in the middle of the night?
Thank you