My Aggie, who turned 13 in December has lost some weight too. She just had a dental, lost no teeth and her blood work was fine.
She eats 4 cups a day plus treats, runs top speed down the back yard, play bows to me all the time and generally acts like she's 3 instead of 13!!
Losing weight must just be a sign that she's getting older in years, but certainly not in spirit!