Depressed Dog? How can you tell?

My girl Bella is now 11 months old. Lately she doesnt seem to be her self. She eats sporadically. Shes not unhealthy though. She'll pick at her food throughout the day. I figured it wasnt a big deal since her health and weight are good. Last night she just seemed sad. Now I know she had a hard morning cause we went out for a walk and it turned out to be much hotter and more humid that I thought so we cut it short and came home. But in the evening she just didnt seem to care about anything. She didnt care to be petted. Didnt want to go outside. Didnt act like her usual ball of energy. I thought maybe the heat just got to her more than usual. Again, this morning, dh let her out of her crate to go outside and she refused to go outside. She just got on the couch and laid her head down. She has acted this way before but not often and not for a long period of time. I can tell when shes too hot, or tired and I can usually pinpoint whats going on with her. If shes depressed, what could it be from? The vet has given her a clean bill of health. Maybe shes just getting older and calming down? If so, I didnt expect that for a couple more years. If you have any thoughts or if your dog has acted this way before please let me know. Maybe I am worried for nothing.