My dog's eating habits?

Lately, the past week, my black labrador retriever has not been eating her food. We give her about 1 cup of dry Beneful (healthy skin and coat) in the morning and once at night. Usually she eats everything in her bowl super fast... but now she avoids eating i guess until shes really really hungry. We switched her food not too long ago because they didnt carry the Beneful stuff, and thats when she slowed down eating...but now we switched back she still isnt eating. She still eats her treats and she BEGS for ham when ever its out, yet she wont eat her dog food. She, like i said, is a black lab, small for her breed, about 60-70 lbs... a tiny beet beefy for her size.... and shes hmmmm 11 now. This has been going on for about two weeks or less.... PLZ give me suggestions!!!! (i am soo nervous about her... her name is Kernan)