I'm so sorry for this. I've been there before as well. Confusion is a good word. My boy was 11 and I noticed he was slowing down, but he still ate, still played in the yard, seemed normal. Late one night I noticed he wouldn't lay down. I made him do it, and he couldn't breathe. He'd stand right back up. A trip to the all night vet showed his lungs were almost full of cancerous tumors.
As it was explained to me, dogs will go to great lengths to hide the fact that they are not feeling well; they don't want their people to think they are sick. I felt guilty for a long time that I didn't know my boy had cancer. He just didn't let it show until it overcame him.
It's possible your girl was not feeling her best, but put on her best face for you. That's the love dogs have for us.
