Mine also looks unhappy now if it is hungry. I haven't checked to see if having no hit points makes them look unhappy, but that is another possibility.
"Oh, better far to live and die/Under the brave black flag I fly/Than play a sanctimonious part/With a pirate head and a pirate heart."
I've noticed this trend a lot when a pet becomes hungry for a prolonged period of time. For instance, my Ixi was looking fine while I was camped in the magic shop and during one of the page refreshes, she went to sad looking and her hunger level is hungry. Hungry pets will get sad looking, but it just seems to basically be teary eyes and nothing else really seems to change.
Nuggets of Wisdom
*When a one-armed man tells you not to juggle chainsaws, you'd better listen
*Sticking your tongue out to catch snowflakes is fun, catching hypothermia is not
Its might be sick or hurgery. Or one time my neopet was crying because she got bored!! So try those if they dont work come back i bet theres going to be more answers