How is the crop this morning? The crop is a muscle that can become stretched over time. But a few days will not stretch it beyond it's ability to return back to normal. I have a hen that is very prone to sour crop, and I have to watch what she eats constantly. The first case she ever got was when she was 6 months old. This crop issue lasted for 2 FULL months, of nearly wearing a baseball around her neck, that is how big her crop was. She nearly died and it wasn't until I figured how not only how to feed her, but how to medicate her, did she survive.
Well after this incident, I was sure she was going to go pendulous in the crop. But it did return back to normal. Every few months after this, she continued to get sour crops and I believe that she does have some sort of digestive issue. I have to watch what I feed her....not too much scratch seeds, hard to break down foods, sweet foods like berries or watermelon, no grass, hay, etc...
I have finally figured out how to care for this bird. She is still prone to it, but gets it less frequently.
Now, if the crop has really gone bad and you do not get it under control after a week, then you can turn to fungal medicines. I keep plenty of it on hand for her and if my hen's sour crop is not gone in 3 days, I usually turn to meds. (I would never do this for any other bird, but I know her track record).
I use the vaginal cream, Clotrimazole, or Gyne-Lotrimin. I know, doesn't sound right. LOL But this ingredient as well as the ingredient in Monostat is used for fungal/yeast infections in humans and is also used for oral thrush in babies. So, it is ok to use this stuff.
I load up about 1 1/2 cc's in a syringe, (stick the tip of the syringe, without needle of course, in the tube and squeeze while opening the plunger on syringe) and squirt this down the throat 3 times a day. This stuff will speed up the healing of the sour crop, which actually IS a yeast infection, and can heal the crop up in a week.
You will need to decide how to treat your bird and when to let her out with the girls. Doesn't sound like there is anything too bad for her crop in your yard, and maybe she has something else going on other than eating something that is not agreeing with her. I would advise that you keep ACV in all the chickens water every day for a week, 2 or 3 tablespoons per gallon of water, (until she gets the crop flora back to normal). Keep up with the plain yogurt. You can always let her out for a few hours a day and then re-cage her so she doesn't get into much. You will need to figure out what is causeing her crop to go bad and if she becomes prone to sour crop, work with her from there. I check my hen's crop each morning while still on the roost bar to keep tabs on her condition.
Good luck!