My dogs attitude has changed?

We adopted my blue heeler mix as a puppy about 4 years ago. When she was still 1-2 she had the most playful personality. If you took one of her toys and threw it in the pool she would jump right in without hesitation. I would walk outside and call her name and she would come running at me with her mouth open and "smiling".Now at the age of almost 4, she no longer does any of those things. She doesn't like to swim in the pool even when its a hot 100 degree summer day. She no longer likes to play with toys. She never comes to me like she used too. Se never answers to her name and what saddens me the most is that she does not "smile" unless i rub her between her ears. What has happened to my dog and how do i bring her back to her happy and playful self.