Sorry to hear that mate. You did the right thing putting her to sleep. It broke my heart loosing the dogs I have so far but as long as you gave her a good life then that's the main thing.
As for what was wrong with her, I was going to suggest cancer. My first dog developed cancer, we knew he had it and he was getting chemo tablets but it spread throughout his body and eventually went into his stomach, he'd eat everything in sight, drink loads and then be sick. In the last day he wouldn't lie down because his stomach was too sore so he walked around the house. We got him put to sleep then because we knew it was time.