Is this sour crop?

I have this 2 months old chicken. She's been very happy and active, jumping on my back every time I went inside their coop. But yesterday she was different. All of a sudden she looked skinnier and lethargic. Today she also had a weird posture. She is slightly bending forward, as if her crop is pulling her down. I didn't see her eat all day. She does seem to drink more than the others though.

Any tips treating this are welcomed, but don't recommend medication since I'm not from America and most probably I won't find it. I heard that apple cider vinegar can make things worse. I would do the upside down vomiting thing but I'm scared that this isn't sour crop and it's gonna hurt her.