Check crops first in the morning only, before they have eaten or drank anything, this is the only time the crop should be empty. During the day they will be full so it's impossible to judge the crop. Olive oil really isn't good for chickens, some plain unsweetened yogurt or probiotics are better. If she definitely has a full crop in the morning, some Acidified Copper Sulfate in the water for a week will clear up any slow or sour crops. The red skin over the crop has nothing to do with what's going on inside the crop.
Yikes, thank you for the warning on the olive oil. Thankfully I only gave her one dose so far, about 0.3mL this morning.
I'm going shopping tonight, so I'll pick her up some yogurt (I am non-dairy, so I never have it in the house!). I'm encouraged by her appetite, but I always freak out a bit when I encounter something I haven't seen before. I really appreciate the replies!