When my friends cat did this it was because her kids sprayed sour candi in his mouth. When my cat did this same thing he is an indoor/outdoor fella and must of ate something?? The coughing up foam was followed by bad diarrhea, some throwing up and not eating or drinking over a long weekend. Being unable to take him to a Vet because of the longweekend, I forced feed him water and used an anti- diarrhea I picked up at petco called naturvet Diarrhea aid. This worked wonders and he was back to his old self the next morning. I would of taken him to the Vet that day if it had not worked so well. Its been a few weeks and he's been fine...other then getting ticked at me for not letting him out as much.
I have to question the "no poisons in home". There are so many things that are toxic to cats that you would never even think about. Houseplants might seem harmless but a large list of them are toxic to cats. http://www.cfainc.org/articles/plants.html Also, any cleaning solution that you use for your floors, etc. can get on a cat's paws and be licked off. White foam usually means there is nothing else in their stomach to bring up. But you said she was eating and using the litterbox ok? Kitties can get occasional flu like symptoms. But they usually stop eating and act a little lethargic for a day or so. If this goes on for more than a day or two or if kitty seems to be in distress, please call the vet. They can become dehydrated quickly and kitty may need fluids and some medicine to get her well.
"One cat just leads to another" - Ernest Hemingway