It's the crop all chickens have it.. I had trouble with mine and these steps helped me to determine if it was infected. When chickens eating something that doesn't digest fully is impacted crop, it usually is due to long grass, hay, or pine shavings. Sour crop is basically a yeast infection in the chickens crop.
Tonight or tomorrow separate you hen from the flock and take all food and water away in the morning before you give her any food or water see if the crop is still squishy, if so she has sour crop. And I will tell you what you do with that if she has it