My chihuahua's appetite is lacking... Whats wrong with her? =(?

I have a 5 year old chihuahua, she use to do a dance when it was feeding time and gobble it down... but for over a month now, she looks at it and walks away. She eventually eats it, but it takes all day. She doesn't have diarreah or vomiting and her behavior is fine... Should I be worried?? Any ideas whats wrong??