Sour crop is a yeast infection, which is why apple cider vinegar works so well for it.
It sounds like you're on top of things now, but a good way to know for sure is to smell their breath. If it smells yeasty, like a sourdough starter, it's sour crop. Also, just like starter yeast, it produces air - you can generally feel/hear it squishing in the crop when checking it out. It can grow up into the throat where you can see it at the back of their mouth sometimes, or go the other way and be noticeable around the vent if the infection is in the digestive tract. (would be white in color on both accounts)
A puffy crop can also be irritated and swollen from other things too - like eating something inappropriate that's caused a scrape or sharp poke.
My silkie personally has a very small crop which is usually rock-hard being jam packed with food all the time. Aka, silkie crops can be a bit weird. :/