I had something similar happy to my Lucy when she was a pup. Of course I paniced and called the Vet the minute it happened!! I think Penny may be getting sick from an empty stomach. Try giving her a "snack" before bed. A little kibble saved from her dinner portion or a biscuit or two from the treat bucket. My Lucy is 3 now and will vomit that frothy green stuff if she hasn't eaten in 12 hours.
Good Luck. Let me know if it works.