Many reasons might cause your cat to feel nauseous, such as eating new food, eating human food, pregnancy, gastritis, motion sickness, etc. If she is feeling nauseous, she might foam at the mouth.
If a cat’s stomach is empty for a longer period of time, a larger amount of gastric secretion accumulates in it. When this happens, her gastric mucosa is irritated, leading to all the substance from her stomach coming out through mouth in the white foam form.