If it's really got you worried, call your vet. It will give you peace of mind!
I had this happen with Missy and I called the Vet. He told me that if she is still eating treats, and she'll eat cheese or favorite foods, it was most likely her trying to con me into giving her what she wanted.
I did what Mangus did. I went to the pet food store, got as whole bunch of samples of great foods and tried them all!
We ended up going with the Stella & Chewies and she's been eating fine since.
Hope this helps!
I know it's so stressful when these tiny babies don't want to eat! They loss weight so fast!