It could just be the switch in food. I know when I switched food, Allen had some problems. Whats the new food? Sometimes they have a red dye on the food that would look pink in the stomach. It probably smells bad because of the stomach acid. Allen vomited last week that smelled terrible -- thousand times worse than a basic hairball. I thought I was going to pass out form the smell

What do you have in the house that is pinkish? I have to hide tissues because Abby will eat them and sometimes they are pink, so maybe she ate some tissues, or found something similar. Never undersestimate what a cat will find to eat.